RFR: CODETOOLS-7903041: jcstress: Provide explicit StressSeed when available

[Aleksey Shipilev]

JDK-8252219 added the StressSeed flag that can be used to specify the seed explicitly. This is handy for reproducing the test failures.

In JCStress failing case, JVM would not dump the StressSeed anywhere (compilation log is not captured by default), so just randomizing the seed explicitly would achieve the same effect. When test crashes, users can then use --jvm(Prepend)Args to set the seed back.

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

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jcstress/pull/100/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=100&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903041
  Stats: 39 lines in 1 file changed: 34 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/100.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/100/head:pull/100

PR: https://git.openjdk.java.net/jcstress/pull/100