Fix threading issues in tests caught by tsan (data race etc)

[iphydf]

See the tsan circle ci build. We have some data races and broken lock handling.

[hugbubby]

What does the tsan build test for that wasn't being looked at?

[iphydf]

It tests for data races, deadlocks, etc. All kinds of threading issues. The code happens to work, but it's probably undefined behaviour.

[hugbubby]

Threading issues in what, though? The tox creation process?

[iphydf]

There are currently no threading issues. TSAN is a blocking check on CI.