RFR: 7904016: preserve XAUTHORITY env variable

[Jaikiran Pai]

Can I please get a review of this change which enhances the jtreg tool to preserve the `XAUTHORITY` environment variable, if set, when launching a test?

This was requested in https://bugs.openjdk.org/browse/CODETOOLS-7904016.

A new self test has been introduced to verify this and other environment variables in a test. The documentation has also been refreshed to list the up-to-date environment variables that are preserved by jtreg.

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Commit messages:
 - 7904016: preserve XAUTHORITY env variable

Changes: https://git.openjdk.org/jtreg/pull/279/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=279&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904016
  Stats: 110 lines in 7 files changed: 98 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/jtreg/pull/279.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/279/head:pull/279

PR: https://git.openjdk.org/jtreg/pull/279