RFR: 7903838: apidiff: Update Vintage JUnit test

[Jonathan Gibbons]

Please review a trivial fix to upgrade a test to use the recommended `@Test` annotation.

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Commit messages:
 - CODETOOLS-7903838: apidiff: Update Vintage JUnit test

Changes: https://git.openjdk.org/apidiff/pull/13/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=13&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903838
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/apidiff/pull/13.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/13/head:pull/13

PR: https://git.openjdk.org/apidiff/pull/13