RFR: 8266002: vmTestbase/nsk/jvmti/ClassPrepare/classprep001 should skip events for unexpected classes

[Alex Menkov]

Class loading may cause loading of some other system/internal classes (for example loading of java.util.concurrent classes when an other thread loads some classes concurrently).
The fix updates ClassPrepare test so it skip events for "unexpected" classes.
Also it adds a testcase to verify that classes which are explicitly loaded on other thread do not generate ClassPrepare event on the test thread.

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Commit messages:
 - ClassPrepare test fix

Changes: https://git.openjdk.java.net/jdk/pull/3732/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3732&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266002
  Stats: 81 lines in 2 files changed: 47 ins; 16 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3732.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3732/head:pull/3732

PR: https://git.openjdk.java.net/jdk/pull/3732