RFR: 7903047: Make JUnit runner use the JUnit Platform Launcher

[Christian Stein]

Make JUnit runner use the JUnit Platform Launcher

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

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Commit messages:
 - Target Java SE 11
 - 7903047: Make JUnit runner use the JUnit Platform Launcher

Changes: https://git.openjdk.java.net/jtreg/pull/78/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=78&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903047
  Stats: 45 lines in 2 files changed: 42 ins; 1 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