RFR: 7903800: apidiff: fix test docs and Makefile

[Jonathan Gibbons]

Please review a trivial fix to address some "TestNG leftovers" from the change to use JUnit

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Commit messages:
 - 7903800: apidiff: fix test docs and Makefile

Changes: https://git.openjdk.org/apidiff/pull/3/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=3&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903800
  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/apidiff/pull/3.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/3/head:pull/3

PR: https://git.openjdk.org/apidiff/pull/3