RFR: 7903884: jtreg -nativepath should support relative path

[Jaikiran Pai]

Can I please get a review of this change which addresses https://bugs.openjdk.org/browse/CODETOOLS-7903884?

jtreg supports the `-nativepath` option. When set, this represents the directory in which native libraries can be located by a test. In one part of the code, jtreg uses this value to configure the operating system specific library path environment variables (for example `DYLD_LIBRARY_PATH` environment variable on macos). The issue here is that when setting this value, the path that was provided to `-nativepath` isn't converted to an absolute path. That causes native library lookup failures when a test requiring those native libraries is run.

The commit in this PR addresses the issue by making sure that the path is converted to an absolute path when setting it on the `RegressionParameters`. This then allows the rest of the jtreg code to consistently use an absolute path for that option.

A new self test has been introduced to verify this behaviour. This and the existing self tests continue to pass with this change.

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Commit messages:
 - 7903884: jtreg -nativepath should support relative path

Changes: https://git.openjdk.org/jtreg/pull/278/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=278&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903884
  Stats: 27 lines in 2 files changed: 23 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jtreg/pull/278.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/278/head:pull/278

PR: https://git.openjdk.org/jtreg/pull/278