FR: Consider running --chain-async-frames on failure only

This might be the best of both worlds. 99% of tests don't fail (i.e. are running on CI).

Example workflow:

1. `pub run test`: All tests passed

No chaining on frames necessary, since there are no failures. Green.

2. `pub run test --chain-async-frames`: All tests passed or tests failed

Always chain frames. Not by default

3. `pub run test`: 1 or more tests failed

The package (`pkg/test`) catches tests with at least 1 uncaught exception/failure, mutes the failure, and then runs *again* with chaining to get a better stack trace.

We could conditionally enable this like --chain-async-frames-on-failure to try it out.

Start in the `pkg/test` package and compare the behavior of `pub run test` with `pub run test --chain-async-frames`. Determine whether the proposed retry-on-failure workflow fits the package's test execution flow, and verify that failures receive chained stack traces without changing the default behavior for passing tests.

Written by the indexing model from the issue text.