[jdk17u-dev] RFR: 8244602: Add JTREG_REPEAT_COUNT to repeat execution of a test

[Aleksey Shipilev]

Unclean backport to improve testing. It does not apply cleanly, because there is a minor conflict in `SINGLE_KEYWORDS` definition due to absent [JDK-8274170](https://bugs.openjdk.java.net/browse/JDK-8274170).

Additional testing:
 - [x] `sanity/IntermittentTest.java` behaves as in original PR

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Commit messages:
 - Backport b2638e5efd3c2b1abe790ab59baf28afa308614f

Changes: https://git.openjdk.java.net/jdk17u-dev/pull/52/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u-dev&pr=52&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244602
  Stats: 28 lines in 3 files changed: 27 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17u-dev/pull/52.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u-dev pull/52/head:pull/52

PR: https://git.openjdk.java.net/jdk17u-dev/pull/52