RFR: 7904071: JTReg should report stats of test collections

[Christian Stein]

Please review this change accumulating the count numbers of TestNG's and JUnit's text report files and prints them after a test run as follows:

> `Framework-based tests: 188, TestNG: 104, JUnit: 84`

Note that an internal, yet public, API of `SummaryReporter` was changed to be package-private now.

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Commit messages:
 - 7904071: JTReg should report stats of test collections

Changes: https://git.openjdk.org/jtreg/pull/283/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=283&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904071
  Stats: 32 lines in 2 files changed: 6 ins; 17 del; 9 mod
  Patch: https://git.openjdk.org/jtreg/pull/283.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/283/head:pull/283

PR: https://git.openjdk.org/jtreg/pull/283