RFR: 8285295: Need better testing for IdentityHashMap [v3]

[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`.

Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:

  Improve wording of comment.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8354/files
  - new: https://git.openjdk.java.net/jdk/pull/8354/files/d66ad0b8..bf4af51f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8354&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8354&range=01-02

  Stats: 6 lines in 1 file changed: 0 ins; 1 del; 5 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