RFR: 7903047: Make JUnit runner use the JUnit Platform Launcher [v12]

[Christian Stein]

> Make JUnit runner use the JUnit Platform Launcher
> 
> See https://bugs.openjdk.java.net/browse/CODETOOLS-7903047

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Add support for `runWithJUnit4` flag

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

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/78/files
  - new: https://git.openjdk.java.net/jtreg/pull/78/files/223af3c2..5454a561

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=78&range=11
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=78&range=10-11

  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/78.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/78/head:pull/78

PR: https://git.openjdk.java.net/jtreg/pull/78