We should improve the error message on string matching in flutter_test.

We have been looking at how to best roll in updates to date localizations that have (in a few cases) updated typical whitespace (U+0020) to be NBSP (non breaking space, U+202F). The localizations team confirmed this is WAI, and in some locales (like French) is a required typography rule.

We have found users' might experience this update like this:

Which would be super frustrating to debug! We should improve the error message here to specify what the whitespace character is in the error message.

Separately we also discussed that in some cases (maybe not this particular case) users might like a test method that allows them to match strings without attention paid to whitespace, tracked in #177478

A little bit more context on this from the localizations team is available in b/442629033