RFR: 8263470: Consolidate copies of getClassBytes in various tests

[Coleen Phillimore]

See issue for details. This also moves ClassUnloadCommon and ClassWithManyMethodsClassLoader to jdk/lib/test/classloader and puts it in a package with other class loader test utility functions. I left getClassBytes in ClassunloadCommon, because most tests that use it also need to import ClassUnloadCommon.

Tested with tier1-3.

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Commit messages:
 - 8263470: Consolidate copies of getClassBytes in various tests

Changes: https://git.openjdk.java.net/jdk/pull/2954/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2954&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263470
  Stats: 460 lines in 36 files changed: 170 ins; 215 del; 75 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2954.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2954/head:pull/2954

PR: https://git.openjdk.java.net/jdk/pull/2954