RFR: 7903390: Summary reporter getter not thread-safe

[Christian Stein]

Prevent `ConcurrentModificationException` by making the two static methods that populate the instance map `synchronized`.

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Commit messages:
 - 7903390: Summary reporter getter not thread-safe

Changes: https://git.openjdk.org/jtreg/pull/139/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=139&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903390
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/139.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/139/head:pull/139

PR: https://git.openjdk.org/jtreg/pull/139