RFR: 8273821: Limit parallelism for sanity/client/SwingSet tests

[Aleksey Shipilev]

If you run them today on a parallel machine with the usual invocation:

$ CONF=linux-x86_64-server-fastdebug make run-test TEST=sanity/client/SwingSet/

...then a whole lot of these tests would fail. It seems to be caused by the overlapping GUI windows that robots fail to reach properly then. It seems these tests need to be run exclusively. 

@mrserb, maybe I am missing something? How do you run these tests? 

Additional testing:
 - [x] `sanity/client/SwingSet/` tests now pass on TR 3970X

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5533/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5533&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273821
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5533.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5533/head:pull/5533

PR: https://git.openjdk.java.net/jdk/pull/5533