errors running nox with PARALLEL_FRAGMENT_TESTS

[vchudnov-g]

When I run nox tests sequentially, they succeed, but when I run them with PARALLEL_FRAGMENT_TESTS, they execute much faster but the actual testing infrastructure (I don't think it's the test themselves) fails.

For example, when I run PARALLEL_FRAGMENT_TESTS=true nox -r -s fragment-3.13, the trailing part of the output looks as follows (notice the lines about pip install failing):

nox > python -m pip install /tmp/tmptrjzmf63 -e . -qqq -r /tmp/tmptrjzmf63/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpheqll1m5 -e . -qqq -r /tmp/tmpheqll1m5/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpa680tu46 -e . -qqq -r /tmp/tmpa680tu46/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpvub8x5yk -e . -qqq -r /tmp/tmpvub8x5yk/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpiz8pix1d -e . -qqq -r /tmp/tmpiz8pix1d/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp8sjnwx4l -e . -qqq -r /tmp/tmp8sjnwx4l/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp8iviuiz3 -e . -qqq -r /tmp/tmp8iviuiz3/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpy3zosy98 -e . -qqq -r /tmp/tmpy3zosy98/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpgjc2d73n -e . -qqq -r /tmp/tmpgjc2d73n/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp3nnytdtl -e . -qqq -r /tmp/tmp3nnytdtl/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp6qju46hr -e . -qqq -r /tmp/tmp6qju46hr/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpyis1yqkr -e . -qqq -r /tmp/tmpyis1yqkr/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpo2dqvtyv -e . -qqq -r /tmp/tmpo2dqvtyv/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpz_s3aimx -e . -qqq -r /tmp/tmpz_s3aimx/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp9ss2cqzu -e . -qqq -r /tmp/tmp9ss2cqzu/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpvo8vap99 -e . -qqq -r /tmp/tmpvo8vap99/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp0ye9o992 -e . -qqq -r /tmp/tmp0ye9o992/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp_c3dp7xe -e . -qqq -r /tmp/tmp_c3dp7xe/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpbn1d2ge9 -e . -qqq -r /tmp/tmpbn1d2ge9/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmp3879pfck -e . -qqq -r /tmp/tmp3879pfck/testing/constraints-3.13.txt
nox > python -m pip install /tmp/tmpa77tbcwt -e . -qqq -r /tmp/tmpa77tbcwt/testing/constraints-3.13.txt
nox > py.test --quiet --cov-config=/tmp/tmp3nnytdtl/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp3nnytdtl/tests/unit
nox > Command python -m pip install /tmp/tmpy3zosy98 -e . -qqq -r /tmp/tmpy3zosy98/testing/constraints-3.13.txt failed with exit code 1:
nox > py.test --quiet --cov-config=/tmp/tmp9ss2cqzu/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp9ss2cqzu/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmpiz8pix1d/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpiz8pix1d/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmpbn1d2ge9/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpbn1d2ge9/tests/unit
nox > Command python -m pip install /tmp/tmp8sjnwx4l -e . -qqq -r /tmp/tmp8sjnwx4l/testing/constraints-3.13.txt failed with exit code 1:
nox > py.test --quiet --cov-config=/tmp/tmp3879pfck/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp3879pfck/tests/unit
nox > Command python -m pip install /tmp/tmpa680tu46 -e . -qqq -r /tmp/tmpa680tu46/testing/constraints-3.13.txt failed with exit code 1:
nox > py.test --quiet --cov-config=/tmp/tmp0ye9o992/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp0ye9o992/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmpvo8vap99/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpvo8vap99/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmp6qju46hr/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp6qju46hr/tests/unit
nox > Command python -m pip install /tmp/tmpz_s3aimx -e . -qqq -r /tmp/tmpz_s3aimx/testing/constraints-3.13.txt failed with exit code 1:
nox > py.test --quiet --cov-config=/tmp/tmpheqll1m5/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpheqll1m5/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmpyis1yqkr/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpyis1yqkr/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmp8iviuiz3/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp8iviuiz3/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmpgjc2d73n/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpgjc2d73n/tests/unit
nox > Command python -m pip install /tmp/tmpvub8x5yk -e . -qqq -r /tmp/tmpvub8x5yk/testing/constraints-3.13.txt failed with exit code 1:
nox > py.test --quiet --cov-config=/tmp/tmpo2dqvtyv/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpo2dqvtyv/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmpa77tbcwt/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmpa77tbcwt/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmptrjzmf63/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmptrjzmf63/tests/unit
nox > py.test --quiet --cov-config=/tmp/tmp_c3dp7xe/.coveragerc --cov-report=term --cov-fail-under=100 /tmp/tmp_c3dp7xe/tests/unit
nox > Session fragment-3.13 failed.