Only loading one test file

[unional]

I encounter this with the combination of the following:

karma-jspm: 2.2.0
jspm: 0.17.0-beta.22
karma: 0.13.22 (not latest due to #162)
karma-tap: 1.0.4 (not latest due to #162)

It happens to some of my projects and not others. After looking into it, I've identified the issue.
It only happens to some of my project due to timing, i.e. when you have multiple test files, some of them could have missed.

filing PR.