Enable concurrency in CI tests

[TzviPM]

As part of dart-archive/ffigen#491, we discovered that some tests could fail with default concurrency of 5, but when run individually or with --concurrency=1 they passed.

We thus added --concurrency=1 to the tests and opted to follow up with solving the thread safety in this issue.

[dcharkes]

@liamappelbe This will be a blocker to merging FFIgen into the dart-lang/native workspace.

[dcharkes]

This also trips up agents when they run dart test with tests that are flaky, triggering the LLM to think that it needs to fix bugs.