RFR: 8300727: java/awt/List/ListGarbageCollectionTest/AwtListGarbageCollectionTest.java failed with "List wasn't garbage collected"

[Alexey Ivanov]

The test has become unstable recently, there were quite a few failures, on Windows mostly. I was lucky enough to find a host where the test failed consistently.

I call `System.gc()` directly as suggested in comments to the bug. I used `PhantomReference` instead of `WeakReference`.

Now the test calls `System.gc()` in a loop and waits for the reference to be enqueued. In majority of cases, the test exits the loop at the second attempt.

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Commit messages:
 - 8300727: java/awt/List/ListGarbageCollectionTest/AwtListGarbageCollectionTest.java failed with "List wasn't garbage collected"

Changes: https://git.openjdk.org/jdk/pull/12594/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12594&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300727
  Stats: 53 lines in 1 file changed: 22 ins; 12 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/12594.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12594/head:pull/12594

PR: https://git.openjdk.org/jdk/pull/12594