Run thread-safety checks when Julia is using threads

[omus]

Digging into #386. Seems like these failures are less random than they first appeared. Running the thread tests in a separate Julia process result in stack traces being lost and only allow for speculative debugging. By enabling threads on the CI and only running the tests when threads are enabled we should be able to get useful information to address the root of the problem.

[codecov-commenter]

Codecov Report

Merging #387 (a7ff9f8) into master (bab4798) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #387   +/-   ##
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  Coverage   95.12%   95.12%           
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  Files          35       35           
  Lines        1743     1743           
=======================================
  Hits         1658     1658           
  Misses         85       85           

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[omus]

I've subverted the thread-safe cache to validate that these thread-safety tests still fail with this change.

[omus]

Tests still pass even without the thread-safe cache. I have managed to produce a thread-safety failure locally so I think the best way to proceed is to merge this PR (with the proper thread-safe cache) and hopefully I can get some additional information in regards to #386