Refactor watcher_test into integration_tests.

[davidmorgan]

Watcher tests:

• Convert many tests to equivalent e2e tests. Rename watch_command_invalidation_test -> watch_command_test and add to that.
• Drop complex invalidation tests as those are best covered by unit tests.
• Drop pubspec.yaml "delayed read" test as that behaviour will be removed soon.

BuildRunnerTester:

• Improved logging around waiting for process output
• Add timeout when waiting for process output
• Stop using TestProcess as there is no way to stop it writing the whole process output when the process exits, just use Process
• Add expectNoOutput

[github-actions]

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Changelog Entry ❗

Package	Changed Files
package:build_runner	build_runner/pubspec.yaml

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