RFR: 8252713: jtreg time out of CtrlASCII.java seems to hang the Xserver.

[Alexander Zvegintsev]

This test needs more stabilization:

- The test can be easily hang by replacing <br> iteration on `keycharHash.elements()` with `keycharHash.forEach((k, v) -> punchCtrlKey(robot, v));` 
I do not see any reason why we need a synchronized `Hashtable`, so switched to `HashMap`
Looks like it might be a reason of original test hanging.
- `keyRelease`, `dispose` calls wrapped with finally block
- Just to be safe added timeout `@run main/timeout=600` ( none of the CI test runs took too long thought)

CI testing looks good on all platforms.

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Commit messages:
 - stabilize
 - clean up
 - Hashtable to HashMap

Changes: https://git.openjdk.org/jdk/pull/11214/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11214&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8252713
  Stats: 217 lines in 2 files changed: 58 ins; 55 del; 104 mod
  Patch: https://git.openjdk.org/jdk/pull/11214.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11214/head:pull/11214

PR: https://git.openjdk.org/jdk/pull/11214