RFR: 7904106: TestNativePath: intermittent test failures due to different test targets using same work dir

[Vladimir Petko]

Both agentvm and othervm targets use the same work path `$(BUILDTESTDIR)/nativepath.othervm/work`. This causes intermittent test failures when the tests are run in parallel. 

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Commit messages:
 - fix: use unique test output paths

Changes: https://git.openjdk.org/jtreg/pull/298/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=298&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904106
  Stats: 19 lines in 1 file changed: 0 ins; 1 del; 18 mod
  Patch: https://git.openjdk.org/jtreg/pull/298.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/298/head:pull/298

PR: https://git.openjdk.org/jtreg/pull/298