RFR: 7903829: apidiff: update TestNG reference to JUnit

[Jonathan Gibbons]

Please review some simple doc fixes to README files.

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Commit messages:
 - CODETOOLS-7903829: apidiff: update TestNG reference to JUnit

Changes: https://git.openjdk.org/apidiff/pull/7/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=7&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903829
  Stats: 19 lines in 2 files changed: 3 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/apidiff/pull/7.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/7/head:pull/7

PR: https://git.openjdk.org/apidiff/pull/7