RFR: 8290531: Loom: Parallelize a few tests more deeply

[Aleksey Shipilev]

Some of the newly added Loom tests are long-running, and thus they delay the completion of otherwise very parallel tier1/jdk_loom. We can parallelize them a bit better to avoid these testing stalls. 

Improvements on Linux x86_64, TR 3970X, `jdk_loom hotspot_loom`:


# release before
real 4m41.424s
user 24m18.064s
sys 1m16.440s

# release after
real 2m47.769s   ; -40%
user 23m44.622s
sys 1m15.240s


# fastdebug before
real 5m38.078s
user 67m23.516s
sys 1m56.446s

# fastdebug after
real 4m9.249s    ; -26%
user 67m21.940s
sys 1m57.625s

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

Commit messages:
 - More fixes
 - Fix

Changes: https://git.openjdk.org/jdk/pull/9554/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9554&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290531
  Stats: 34 lines in 2 files changed: 19 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/9554.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9554/head:pull/9554

PR: https://git.openjdk.org/jdk/pull/9554