[ffigen] Support concurrent tests

[liamappelbe]

The main fix was to stop setting Directory.current. Luckily this only happened in the withChDir until, so it was easy to remove. For the most part that worked without any other changes. The only exception was the libclang-example test, which was relying on relative file paths in their extra clang command line options.

Also had to stop using Logger.root in the test context.

Fixes #461

[coveralls]

coverage: 81.978% (+0.009%) from 81.969%
when pulling 7ed5dbc on ffigen_concurrent_tests
into 1391b39 on main.

[github-actions]

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[dcharkes]

Sweet to see this long standing TODO being addressed! 😄