RFR: 7903812: Extract property name added with CODETOOLS-7900303 to a string constant

[Dmitry Bessonov]

A string constant can be used for the property "javatest.desktop.testrunprogressmonitor.hidden" introduced by [CODETOOLS-7900303](https://bugs.openjdk.org/browse/CODETOOLS-7900303)

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Commit messages:
 - 7903812: Extract property name added with CODETOOLS-7900303 to a string constant

Changes: https://git.openjdk.org/jtharness/pull/83/files
  Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=83&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903812
  Stats: 14 lines in 3 files changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jtharness/pull/83.diff
  Fetch: git fetch https://git.openjdk.org/jtharness.git pull/83/head:pull/83

PR: https://git.openjdk.org/jtharness/pull/83