RFR: 7903047: Introduce new run action running the JUnit Platform

[Christian Stein]

https://bugs.openjdk.java.net/browse/CODETOOLS-7903047

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Commit messages:
 - Append JUnit Platform's JAR to the build class paths
 - 7903047: Introduce new run action running the JUnit Platform

Changes: https://git.openjdk.java.net/jtreg/pull/34/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=34&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903047
  Stats: 431 lines in 14 files changed: 416 ins; 0 del; 15 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/34.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/34/head:pull/34

PR: https://git.openjdk.java.net/jtreg/pull/34