RFR (S): 8147075: Rename old GC JTreg tests to the new naming scheme

[Dmitry Fazunenko]

Hello,

would you please review the following test refactoring (renaming):
    test/gc/6581734/Test6581734.java -->test/gc/cms/TestMBeanCMS.java
    test/gc/6845368/bigobj.java --> test/gc/TestBigObj.java
    test/gc/7072527/TestFullGCCount.java --> test/gc/TestFullGCCount.java

CR:  https://bugs.openjdk.java.net/browse/JDK-8147075
Webrev: http://cr.openjdk.java.net/~dfazunen/8147075/webrev.00/

I did also some very minor changes in the code, but the original logic 
of the tests is preserved.
Tested: locally + RBT is in progress.

Test renaming is a bad practice and should be done only as an exception.
In this case such renaming is quite safe: I searched for the old test 
names in JIRA and haven't found any open CRs.

Thanks,
Dima