Add bool return typehints to protected test helper methods

Might not have found them all.

Seems straight forward.

Not sure if we should do AssertContentTrait here, or do #3129006: Modernize Drupal\KernelTests\AssertContentTrait → / #3227335: Deprecate unused kernel test assertions in AssertContentTrait → instead - this will definitely conflict with those two issues. Going to ping @longwave who was working on both.

catch → credited longwave → .

@longwave in slack said he thinks it's fine for this to go in first.

Committed/pushed to 11.x, thanks!

catch → closed merge request !8511

Automatically closed - issue fixed for 2 weeks with no activity.