RFR: 8285295: Need better testing for IdentityHashMap

[Stuart Marks]

Basic but fairly comprehensive set of tests for `IdentityHashMap`. The patch in the bug report that breaks `IdentityHashMap` now causes several cases in this new test to fail. There's more that could be done, but the new tests cover most of the core functions of `IdentityHashMap`. Unfortunately it seems difficult to merge this with the existing, comprehensive Collections tests (e.g., MOAT.java) because those tests implicity rely on `equals()`-based contract instead of the special-purpose `==`-based contract used by `IdentityHashMap`.

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Commit messages:
 - Refactor contents checking to use checkElements() and checkEntries().
 - Rename some tests.
 - Rename isIdenticalEntry to hasIdenticalKeyValue.
 - Finish writing all pending tests except remove(k,v) and replace(k,v1,v2).
 - Some cleanup and more tests.
 - Initial cut at IdentityHashMap tests.

Changes: https://git.openjdk.java.net/jdk/pull/8354/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8354&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285295
  Stats: 546 lines in 1 file changed: 546 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8354.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8354/head:pull/8354

PR: https://git.openjdk.java.net/jdk/pull/8354