[jdk11u-dev] RFR: 8274523: java/lang/management/MemoryMXBean/MemoryTest.java test should handle Shenandoah

[Aleksey Shipilev]

Unclean backport to improve Shenandoah test. The uncleanliness is due to test block missing for Z (added later by JDK-8240679). I just dropped the `@requires vm.gc != Z` clause from one of the blocks.

Additional testing:
 - [x] Linux x86_64 fastdebug test now passes with Shenandoah

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Commit messages:
 - Backport f8415a9b2f610ed431e6948c8174f6d982e5b31f

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/603/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=603&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274523
  Stats: 13 lines in 1 file changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/603.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/603/head:pull/603

PR: https://git.openjdk.java.net/jdk11u-dev/pull/603