RFR(xs): 8224710: [TESTBUG] dynamicArchive/ExcludedClasses.java test failed in tier6 testing on Windows

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8224710

webrev: http://cr.openjdk.java.net/~ccheung/8224710/webrev.00/

The test failure was observed on Windows once when the loading of the 
base archive failed. A simple fix is in CDSTestUtils::isUnableToMap() to 
account for the failure in loading of the base archive.

I haven't been able to reproduce the bug so far but was able to simulate 
the failure condition by renaming the base archive prior to running the 
test.

Testing: ran the test on Windows and Linux machines many times.

thanks,
Calvin