RFR: 7903047: Introduce new run action running the JUnit Platform [v2]

[Christian Stein]

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

Christian Stein has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'openjdk:master' into 7903047-new-junit-platform-action
 - Append JUnit Platform's JAR to the build class paths
 - 7903047: Introduce new run action running the JUnit Platform

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

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/34/files
  - new: https://git.openjdk.java.net/jtreg/pull/34/files/2543a379..9ee00b8e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=34&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=34&range=00-01

  Stats: 64 lines in 3 files changed: 32 ins; 0 del; 32 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