RFR: 7903872: Option testThreadFactoryPath support relativepath

[SendaoYan]

Hi all,
Currently, the jtreg option `-testThreadFactoryPath` doesn't support relativepath.
I think jtreg can support relativepath for option `-testThreadFactoryPath`, same like to `-jdk` or `-nativepath`.
Change has been verified locally, risk is low.

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Commit messages:
 - 7903872: Option testThreadFactoryPath support relativepath

Changes: https://git.openjdk.org/jtreg/pull/233/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=233&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903872
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/233.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/233/head:pull/233

PR: https://git.openjdk.org/jtreg/pull/233