RFR: CODETOOLS-7903250: RerunTest fails if no display is set

[Jonathan Gibbons]

Please review a simple fix for a jtreg self-test, to set a dummy value for DISPLAY if no value already set.

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Commit messages:
 - CODETOOLS-7903250: RerunTest fails if no display is set

Changes: https://git.openjdk.org/jtreg/pull/101/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=101&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903250
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/101.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/101/head:pull/101

PR: https://git.openjdk.org/jtreg/pull/101