[jdk11u-dev] RFR: 8244602: Add JTREG_REPEAT_COUNT to repeat execution of a test

[Goetz Lindenmaier]

I backport this for parity with 11.0.16-oracle.

I had to resolve RunTests.gmk due to context.

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Commit messages:
 - Backport b2638e5efd3c2b1abe790ab59baf28afa308614f

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/1019/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=1019&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/jdk11u-dev/pull/1019.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/1019/head:pull/1019

PR: https://git.openjdk.java.net/jdk11u-dev/pull/1019