[xbuild]: Fix the tests.

'TestMessageLogger.Count' should not be used in tests to check for the
number of logged messages because it would also includes diagnostic messages.

Extra diagnostic messages should not be considered an error condition; in
fact, we have too few diagnostic messages, not too many.

Use 'TestMessageLogger.NormalMessageCount' and 'TestMessageLogger.WarningMessageCount'
instead.
Fix some warnings in mini-arm.c.