RFR: 8274273: Update testing docs for MacOS with Non-US locale

[Jie Fu]

Hi all,

I'd like to update the testing docs for MacOS with Non-US locale.

Our experiments show that the solution mentioned for Windows with Non-US locale also works for MacOS.
So it would be helpful to mention MacOS explicitly.

FYI: changes in doc/testing.html was made by `make update-build-docs` .

Thanks.
Best regards,
Jie

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Commit messages:
 - 8274273: Update testing docs for MacOS with Non-US locale

Changes: https://git.openjdk.java.net/jdk/pull/5676/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5676&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274273
  Stats: 28 lines in 2 files changed: 8 ins; 1 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5676.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5676/head:pull/5676

PR: https://git.openjdk.java.net/jdk/pull/5676