Sometimes the test suite hangs

[exarkun]

With seemingly increasing frequency, the test suite hangs a couple tests into allmydata.test.test_runner.RunNode?

It seems suspicious that the hang happens on the 2nd test that does a deferToThread(subprocess.Popen, ...)

The subprocess module on Python 2.7 is basically not thread-safe.

[GitHub <noreply@…>]

In 7bda21a/trunk:

Merge pull request #593 from tahoe-lafs/3021.test-suite-hang

Fix one of the test suite hangs.

Fixes: ticket:3021