Update testExecutable signature in documentation

[ValentinVignal]

In flutter/packages#7546 I discovered that the signature of testExecutable should be

Future<void> testExecutable(FutureOr<void> Function() testMain)

to be able to compile the tests for the chrome platform.

This PR updates the documentation.

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[ValentinVignal]

cc @polina-c

[ValentinVignal]

I believe this doesn't need a CHANGELOG entry/version bump/tests ?

[auto-submit]

auto label is removed for dart-lang/leak_tracker/245, due to This PR has not met approval requirements for merging. The PR author is not a member of dart-team and needs 1 more review(s) in order to merge this PR.

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[polina-c]

Yes, no changelog is needed.
Thank you!