RFR: 8202926: Test java/awt/Focus/WindowUpdateFocusabilityTest/WindowUpdateFocusabilityTest.html fails

[Alexander Zvegintsev]

The test is still failing on Ubuntu 16.04 (`window1` and `window2` are not placed in front, so they are not receiving events).

Looks like this was fixed in later releases. I've manually checked this test on Ubuntu 18.04, 20.04, 21.04 and they are not affected. 

We can remove the test from the problem list since we are no longer supporting Ubuntu 16.04.

Automated testing is also green.

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Commit messages:
 - initial

Changes: https://git.openjdk.java.net/jdk/pull/6121/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6121&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8202926
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6121.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6121/head:pull/6121

PR: https://git.openjdk.java.net/jdk/pull/6121