RFR: JCStress Pre-Integration Tests workflow

[Aleksey Shipilev]

This adds the PR checks that verify JCStress is not broken for integration.

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Commit messages:
 - Drop push trigger from pre-integration tests
 - Restore OS matrix
 - Trying again without context braces
 - Trying again with split
 - Shorter jobs for conditional all/custom tests
 - Conditionalize all/custom tests for Windows runners
 - Chasing slow Windows worker times
 - More targeted testing for Windows
 - Trying to please Windows runners again
 - Saner stage names
 - ... and 13 more: https://git.openjdk.java.net/jcstress/compare/c2974120...a6508828

Changes: https://git.openjdk.java.net/jcstress/pull/2/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=2&range=00
  Stats: 46 lines in 2 files changed: 45 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/2.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/2/head:pull/2

PR: https://git.openjdk.java.net/jcstress/pull/2