Test snack bar examples

[NobodyForNothing]

Adds tests to the last two Snack Bar examples as part of #130459. Makes the last example more usable through the use of standard widgets and visual hierarchy. Constraints on options that are not required by the SnackBar contract have been removed (Overflow threshold works on fixed SnackBars).

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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