RFR: CODETOOLS-7903152: On macOS, avoid using /usr/bin/tidy

[Jonathan Gibbons]

Please review a simple makefile change to search for a better version of `tidy` on macOS than that in /usr/bin/tidy.

For compatibility, there is no change in the behavior on other platforms, although it might be reasonable to eventually extend the search to `/usr/local/bin/tidy` on other platforms if needed.

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Commit messages:
 - CODETOOLS-7903152: On macOS, avoid using /usr/bin/tidy

Changes: https://git.openjdk.java.net/jtreg/pull/71/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=71&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903152
  Stats: 12 lines in 1 file changed: 10 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/71.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/71/head:pull/71

PR: https://git.openjdk.java.net/jtreg/pull/71