RFR: 8258415: gtest for committed memory leaks reservation

[Coleen Phillimore]

Add a call to os::release_memory() in the gtest.
Tested with gtest on linux-x64-debug.  tier1 testing in progress to run on other platforms.

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Commit messages:
 - 8258415: gtest for committed memory leaks reservation

Changes: https://git.openjdk.java.net/jdk/pull/1809/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1809&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258415
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1809.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1809/head:pull/1809

PR: https://git.openjdk.java.net/jdk/pull/1809