Consolidate copies of getClassBytes in various tests

There are several tests with the same copy of getClassBytes to read a class file for ClassLoader tests. I've been cut/pasting this copy into new tests. These should call the version in ClassUnloadCommon, which also should be fixed to not depend on the location of the test.classes property.

Also move ClassUnloadCommon.java and ClassWithManyMethodsClassLoader.java from test/hotspot/jtreg/runtime/testlibrary to test/lib/jdk/test/lib/classloader so we don't need @library /runtime/testlibrary and /test/lib.