This PR makes GTK unit tests abort on GTK critical errors. This including GTK internal errors and g_return_if_fail statements added by Flutter. This PR also fixes three such critical errors that have been ignored.

These g_return_if_fail statements have been used as (and practically are) the GTK version of asserts. However, currently these statements only logs to stderr, while the process goes on with a return value 0, making them pretty much ignored.

I've tried to look for a way to only return a non-zero value instead of aborting, but in vain.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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