RFR(S): 8236938: [TESTBUG] JFR event MetaspaceAllocationFailure is not tested

[mikhailo.seledtsov at oracle.com]

Please review this new test for JFR event MetaspaceAllocationFailure.

The test fills the metaspace by generating classes in a loop, until the 
desired event is delivered to the receiver.
By creating a new instance of class loader for each iteration test 
allows metaspace to be cleaned up once MetaspaceAllocationFailure
condition is detected, thus not causing full-blown OOME. However, on 
occasion, it may happen, hence the try/catch clause.
Ran this at least 100 times, works well and reliably.

Also moved the class generator utility from Runtime test library to 
common test library; renamed it to avoid name clash with
another already existing GeneratingClassLoader.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8236938
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8236938.01/
     Testing:
         1. Ran over 100 times on Linux, Windows and MAC: All PASS
         2. Ran all runtime tests that rely on the class loader 
generator runtime library.


Thank you,
Misha