[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 https://github.com/dart-lang/native/issues/461

[coveralls]

coverage: 81.978% (+0.009%) from 81.969% when pulling 7ed5dbcbd6fbb00c4b294b121883004acd541311 on ffigen_concurrent_tests into 1391b39d7c6f9058213adb60117becd9d2dd0c86 on main.

[github-actions[bot]]

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