RFR: JDK-8305416: runtime/Thread/TestAlwaysPreTouchStacks.java failed with "Did not find expected NMT output"

[Thomas Stuefe]

Fix test. Several issues caused the test to fail intermittently (only on Windows for some reason).

The test starts a a child process with -XX:+AlwaysPreTouchStacks. In this child process, it creates n threads with given stack sizes, then expects them to come up successfully, then checks the final NMT output for the expected stack committed size (which should ideally be very close to the stack reserved size, in contrast to a VM started without -AlwaysPreTouchStacks).

The test had several errors:
- since the threads started by the test were non-daemon threads, they were exited before the JVM finished. So their stacks did not even show up in the final NMT report.
- The test threads create log output (via Xlog). If that output gets mixed into the final NMT report, the report parser gets confused. This is what happened on Windows, and this made the error intermittent.

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The new, fixed test:
- creates the test threads as daemon threads, so they stick around till VM death and show up in the final NMT report. We also create more test threads with noticeably larger stacks for them to have a visible impact on NMT-reported stack sizes.
- removed the Xlog output
- The test main thread waits until all test threads have successfully started.
- I removed the testing of different stack sizes since it was not useful
- I made the test stricter: 
   - The total committed thread stack size should be more than 3/4ths of the total committed reserved stack size for the test to succeed. This, in combination with large test thread stacks, is enough to prove that PreTouch is working
   - I also added a santity test for the total reserved stack size to be larger - but not insanely so - than the combined test thread stack sizes to prove that what we see is caused by our test threads.

Tests: manually ran tests on Windows x64 and Linux x64.

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Commit messages:
 - JDK-8305416-runtime-Thread-TestAlwaysPreTouchStacks-fails

Changes: https://git.openjdk.org/jdk/pull/13295/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13295&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305416
  Stats: 56 lines in 1 file changed: 36 ins; 6 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/13295.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13295/head:pull/13295

PR: https://git.openjdk.org/jdk/pull/13295