Error decoding plan output if TF_LOG is used

[dcopestake]

I started encountering the error:

tftest.TerraformTestError: Error decoding plan output: Extra data: line 1 column 5 (char 4)

After some further inspection it turns out this is because I have TF_LOG set to INFO and the additional information output to stdout is throwing off TerraformPlanOutput as it's obviously not JSON.

I guess an easy fix might be to just unset the TF_LOG variable entirely for terraform runs executed by tftest?