RFR: 8272970: Parallelize runtime/InvocationTests/

[Aleksey Shipilev]

Current runtime/InvocationTests/ are long and serial. They take about half an hour on a modern desktop, mostly using a single CPU. They can be parallelized better, at least on sub-test level. This would be important as [JDK-8272914](https://bugs.openjdk.java.net/browse/JDK-8272914) would start to run them in tier3.

Sample run times:


$ time CONF=linux-x86_64-server-fastdebug make run-test TEST=runtime/InvocationTests/

# Before
real	32m2.188s
user	59m12.818s
sys	0m45.612s

# After
real	17m21.110s
user	63m31.851s
sys	0m55.391s


I think we can technically go even deeper, for example parallelising by `packageSet` in the generators themselves, but that would be much more intrusive. I think 17 minutes is already quite good, given other tests running in prospective `hotspot:tier3`.

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Commit messages:
 - 8272970: Parallelize runtime/InvocationTests/

Changes: https://git.openjdk.java.net/jdk/pull/5250/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5250&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272970
  Stats: 278 lines in 6 files changed: 245 ins; 10 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5250.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5250/head:pull/5250

PR: https://git.openjdk.java.net/jdk/pull/5250