RFR: 8286956: Loom: Define test groups for development/porting use

[Aleksey Shipilev]

It would be beneficial to bring over the Loom-specific test groups from the loom repo to aid development/porting work.

https://github.com/openjdk/loom/blob/fibers/test/jdk/TEST.groups#L97-L108
https://github.com/openjdk/loom/blob/6f42923b3342e41d95b262733205283068802b40/test/hotspot/jtreg/TEST.groups#L111-L125

I had to drop `jdk/incubator/concurrent` from JDK definition, because there are no tests in that folder and tests break.

Additional testing:
 - [x] Linux x86_64 fastdebug `hotspot_loom jdk_loom`
 - [ ] Linux AArch64 fastdebug `hotspot_loom jdk_loom`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/8765/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8765&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286956
  Stats: 28 lines in 2 files changed: 28 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8765.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8765/head:pull/8765

PR: https://git.openjdk.java.net/jdk/pull/8765