ScannerTest#testPerformance(): Improve failure reporting.

This test apparently sometimes fails to match "123 " against
the floatPattern constructed at runtime, but I can't reproduce and
can't see why it would. It might be related to an issue inside
ICU that only occurs a rare fraction of the time, or it might
be related to a nonstandard Locale. This CL adds more detail to
the failure case in order to narrow this down.

I've also slightly tweaked the initialization part of the test.

Bug: 150310094
Test: atest CtsLibcoreTestCases:org.apache.harmony.tests.java.util.ScannerTest#testPerformance
      (takes about 15 seconds on a Pixel 2).
Test: To make sure that I didn't have a bug in my String.format(),
      temporarily moved the initialization of String message
      to in front of the try block and checked that the test
      still passed (but then took 21sec, unsurprisingly).
Change-Id: Ib7d29e798515ebd7da474916078f6f3b39fd8a62
1 file changed