RFR: 7904084: Unset javaOpts and jtregOpts in jtreg.sh

[SendaoYan]

Hi all,

In jtreg.sh it will combine all the java options and jtreg options in the `for i in "$@"` loop. SHELL use javaOpts and jtregOpts as global varable by default, this means jtreg.sh will receive the value of javaOpts and jtregOpts outside. I think this unexpected.
To avoid this siutation, I think we should unset javaOpts and jtregOpts before use it.

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Commit messages:
 - 7904084: Unset javaOpts and jtregOpts in jtreg.sh

Changes: https://git.openjdk.org/jtreg/pull/291/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=291&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904084
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jtreg/pull/291.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/291/head:pull/291

PR: https://git.openjdk.org/jtreg/pull/291