RFR: 7903047: Make JUnit runner use the JUnit Platform Launcher [v10]

[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:

  Agent's sources target Java SE release 8

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Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/78/files
  - new: https://git.openjdk.java.net/jtreg/pull/78/files/f402d99b..0cdb4e48

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=78&range=09
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=78&range=08-09

  Stats: 15 lines in 3 files changed: 5 ins; 0 del; 10 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