RFR: 8252196: ZGC: TestUncommit.java fails due to "Exception: Uncommitted too fast" again(2)

[Per Lidén]

This test has failed a few times because the timeAfterAlloc timestamp (the time when uncommit can start to happen) is
taken at the wrong place. We should take this timestamp before we call allocate(), since the allocation could cause
ZPageCache::_last_commit to be set, which will dictate when uncommit can happen. This PR moves and renames
timeAfterAlloc accordingly.

Testing: gc/z/TestUncommit.java executed multiple times on all platforms.

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Commit messages:
 - 8252196: ZGC: TestUncommit.java fails due to "Exception: Uncommitted too fast" again(2)

Changes: https://git.openjdk.java.net/jdk/pull/179/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=179&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252196
  Stats: 3 lines in 1 file changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/179.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/179/head:pull/179

PR: https://git.openjdk.java.net/jdk/pull/179