RFR: JDK-8267371: Concurrent gtests take too long

[Thomas Stuefe]

More than half of the total runtime of the gtests is currently used by just three tests:

- VirtualSpace.os_reserve_space_concurrent_vm
- VirtualSpace.os_virtual_space_concurrent_vm
- os_linux.reserve_memory_special_concurrent_vm

These tests do concurrent tests with 30 threads, with a timeout of 15 seconds each.

Since we run the gtests with several configurations (see hotspot/jtreg/gtests) these numbers multiply, which hurts especially when run as part of tier1. I think we can safely reduce the timeouts, still have good test coverage and recover some of the gtest runtime.

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The patch reduces the timeout to 5 seconds, which reduces the time for one gtest by 30 seconds from 80 seconds down to 50 seconds.

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/4110/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4110&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267371
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4110.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4110/head:pull/4110

PR: https://git.openjdk.java.net/jdk/pull/4110