RFR: 8310187: Improve Generational ZGC jtreg testing [v3]

[Axel Boldt-Christmas]

> The current implementation for testing generational ZGC with jtreg is implemented with a filter on the mode flag `ZGenerational`. Because of this only environments which set this flag explicitly will run most of the tests. 
> So they get missed in Github Actions and for developers running jtreg locally without supplying the `ZGenerational` flag.
> 
> The proposed change here is to introduce two new jtreg requirement properties, `vm.gc.ZGenerational` and `vm.gc.ZSingelgen`. These flags will effectively behave the same as the existing `vm.gc.<GC>` flags but also take the specific ZGC mode in account.
> 
> If no gc flags are supplied to jtreg and the `vm.gc.Z` is true (the build includes ZGC) both `vm.gc.ZGenerational` and `vm.gc.ZSingelgen` will be true.
> 
> If `-XX:+UseZGC` is supplied then both `vm.gc.ZGenerational` and `vm.gc.ZSingelgen` will also be true.
> 
> If `-XX:{+,-}ZGenerational` is supplied then either  `vm.gc.ZGenerational` or `vm.gc.ZSingelgen` be true depending on the flags value.
> 
> And if `vm.gc.Z` is false both `vm.gc.ZGenerational` and `vm.gc.ZSingelgen` will be false.
> 
> This change also splits the relevant tests into two distinct runs for the two modes. And the respective test ids are set to `ZGenerational` or `ZSinglegen` to make it easier to distinguish the runs.
> 
> This also solves the issue that some compiler tests will never run with generational ZGC unless the `TEST_VM_FLAGLESS` is set. This is because the current filter `vm.opt.final.ZGenerational` requires the flag to be explicit, but these compiler tests uses `vm.flagless`. 
> 
> The introduction of  `vm.gc.ZGenerational` and `vm.gc.ZSingelgen` harmonizes the way you specify generational / single gen ZGC test with the way it is done for other gcs with `vm.gc.<GC>`
> 
> To support this feature the Whitebox API is extended with `isDefaultVMFlag` to enable checking if `ZGenerational` is default or not.
> 
> `vm.opt.final.ZGenerational` is still kept because we still have some reasons to filter based on the supplied flags. 
> - `test/hotspot/jtreg/gc/cslocker/TestCSLocker.java` is disabled when running with ZGenerational
> - `test/jdk/java/lang/ProcessBuilder/CloseRace.java` is ran with a different max heap size for ZGenerational, but it is not the intent to dispatch the test to both G1 and generational ZGC if Generational ZGC is not specified.  
> 
> `test/jdk/java/lang/management/MemoryMXBean/MemoryTest.java` was also changed to not filter but instead dispatch. However unsure if this change should be included. The change ...

Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:

  Add explicit id to all Skynet.java @test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14509/files
  - new: https://git.openjdk.org/jdk/pull/14509/files/e571b87f..b262d48b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14509&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14509&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14509.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14509/head:pull/14509

PR: https://git.openjdk.org/jdk/pull/14509