RFR: 8358284: doc/testing.html is not up to date after JDK-8355003

[Kim Barrett]

Please review this trivial change to doc/testing.html, to bring it up to date
with the associated source file doc/testing.md.  The change was produced by
running `make update-build-docs` in an up-to-date local repository.

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Commit messages:
 - update testing.html

Changes: https://git.openjdk.org/jdk/pull/25573/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25573&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358284
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/25573.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25573/head:pull/25573

PR: https://git.openjdk.org/jdk/pull/25573