The intent of package-looping commands is that we always want to run them for all the targeted packages, accumulating failures, and then report them all at the end, so we don't end up with the problem of only finding errors one at a time. However, some of them were using `exitOnError: true` for the underlying commands, causing the first error to be fatal to the test.
This PR:
- Removes all use of `exitOnError: true` from the package-looping commands. (It's still used in `format`, but fixing that is a larger issue, and less important due to the way `format` is currently structured.)
- Fixes the mock process runner used in our tests to correctly simulate `exitOrError: true`; the fact that it didn't was hiding this problem in test (e.g., `drive-examples` had a test that asserted that all failures were summarized correctly, which passed because in tests it was behaving as if `exitOnError` were false)
- Adjusts the mock process runner to allow setting a list of mock result for a specific executable instead of one result for all calls to anything, in order to fix some tests that were broken by the two changes above and unfixable without this (e.g., a test of a command that had been calling one executable with `exitOnError: true` then another with ``exitOnError: false`, where the test was asserting things about the second call failing, which only worked because the first call's failure wasn't actually checked). To limit the scope of the PR, the old method of setting a single result for all calls is still supported for now as a fallback.
- Fixes the fact that the mock `run` and `runAndStream` had opposite default exit code behavior when no mock process was set (since that caused me a lot of confusion while fixing the above until I figured it out).
Add a summary to the end of successful runs for everything using the new looping base command, similar to what we do for summarizing failures. This will make it easy to manually check results for PRs that we know should be changing the set of run packages (adding a new package, adding a new test type to a package, adding a new test type to the tool), as well as spot-checking when we see unexpected results (e.g., looking back and why a PR didn't fail CI when we discover that it should have).
To support better surfacing skips, this restructures the return value of `runForPackage` to have "skip" as one of the options. As a result of it being a return value, packages that used `printSkip` to indicate that *parts* of the command were being skipped have been changed to no longer do that.
Fixes https://github.com/flutter/flutter/issues/85626
Migrates firebase-test-lab to use the new package-looping base command.
Other changes:
- Extracts several helpers to make the main flow easier to follow
- Removes support for finding and running `*_e2e.dart` files, since we no longer use that file structure for integration tests.
Part of https://github.com/flutter/flutter/issues/83413
