RFR: CODETOOLS-7903264: Full support for JUnit tests

[Jonathan Gibbons]

Please review a medium-size patch to upgrade the support for running directories of Unit tests. This is primarily in the jtreg front end, either cloning or sharing and then modifying the support for directories of TestNG tests. The back end already supports direct use of JUnit for `@run junit` and the new code leverages that work.

There is one tweak to the JUnit back-end, to enable the generation of summary reports. To that end, the class `TestNGSummaryReporter` has morphed into `SummaryReporter` with internal subtypes for `JUnit` and `TestNG`. That refactoring was too much for git to recognize, and so the change shows up as "old file deleted; new file added".

Two new groups of tests are added, which are just modified copies of existing TestNG tests, to verify that JUnit support is effectively equivalent to TestNG support.

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Commit messages:
 - update to use JUnit
 - CODETOOLS-7903264: Full support for JUnit tests

Changes: https://git.openjdk.org/jtreg/pull/107/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=107&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903264
  Stats: 852 lines in 26 files changed: 630 ins; 153 del; 69 mod
  Patch: https://git.openjdk.org/jtreg/pull/107.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/107/head:pull/107

PR: https://git.openjdk.org/jtreg/pull/107