RFR: 8213269: convert test/hotspot/jtreg/runtime/memory/RunUnitTestsConcurrently to gtest [v6]

[Mikhailo Seledtsov]

> This is a preliminary review. I would like to get the initial feedback before I proceed with conversion of the remaining tests.
> 
> Here is what I did so far:
>   - created a UnitTestThread and a main test runner, based on gtests with similar needs
>   - moved the original code from HotSpot internals (so called hotspot internal tests: src/hotspot/share/memory/virtualspace.cpp)
>     to the newly created gtest while wrapping it into a TestReservedSpace class. I did not change the code of the test.
>   - removed invocations from whitebox.cpp
> 
> Testing:
>   - ran GTestWrapper on usual platforms - All PASS
>   - ensured that ReservedSpaceConcurrent is in the logs and passed
> 
> After gathering the feedback my plan is:
> Plan:
>   - move the remaining internal Memory/VirtualSpace tests into a gTest
>     - I am thinking about using separate files for each test
>   - create a common file for UnitTestThread and MultiThreadTestRunner to reuse the code

Mikhailo Seledtsov has updated the pull request incrementally with one additional commit since the last revision:

  Using regular brackets in initializer list instead of curly brackets

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2436/files
  - new: https://git.openjdk.java.net/jdk/pull/2436/files/f8db673b..d9f618c2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2436&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2436&range=04-05

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2436.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2436/head:pull/2436

PR: https://git.openjdk.java.net/jdk/pull/2436