update test expectation for newer versions of package:args

[devoncarew]

• update test expectation for newer versions of package:args

A not-yet-published version of package:args will change the exception's message from Could not find an option named "nonexistent" to Could not find an option named "--nonexistent"; this adjusts the test expectation to work for either package:args version.

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[devoncarew]

(this update will allow us to roll the latest package:args into the sdk and google3)

[srawlins]

Thanks!

[devoncarew]

Hmm, it looks like I'm still seeing a failure from https://github.com/dart-lang/dartdoc/blob/main/test/documentation_comment_test.dart#L454; we should relax that check as well.