[8u] RFR: 8081547: Prepare client libs regression tests for running in a concurrent, headless jtreg environment

[Severin Gehwolf]

Hi,

Please review this 8u backport of test-only changes. The JDK 9 patch
didn't apply cleanly, but were manually resolved. The only difference
were context changes in the tests (@modules tag not present in JDK 8)
which made them different enought for the patch to not apply.

Bug: https://bugs.openjdk.java.net/browse/JDK-8081547
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8081547/01/webrev/

Testing: Affected tests on Linux x86_64 with/without -k:headful.
         Worked correctly after patch.

Thanks,
Severin