RFR: 8371698: ZGC: Call GTEST_SKIP when OS is unsupported

[Stefan Karlsson]

We have a ZGC-specific test fixture for our gtests. This fixture bails out if the OS is unsupported. In addition to that, all tests need to bail out as well. It turns out that we missed this bail-out in one of our tests.

I propose that we perform a stronger bail-out in the test fixture to make this error less likely to happen in the future.

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Commit messages:
 - ZGC: Call GTEST_SKIP when OS is unsupported

Changes: https://git.openjdk.org/jdk/pull/28348/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28348&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371698
  Stats: 40 lines in 4 files changed: 9 ins; 31 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28348.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28348/head:pull/28348

PR: https://git.openjdk.org/jdk/pull/28348