RFR: 7903692: JTREG_HOME and JT_HOME can interfere with jtreg self tests

[Jaikiran Pai]

Can I please get a review of this change which proposes to fix the issue noted in https://bugs.openjdk.org/browse/CODETOOLS-7903692?

As noted in that issue, when running the jtreg self tests for the project, if `JT_HOME` (or `JTREG_HOME`) have been configured to point to some specific jtreg installation, then the self tests instead of running against the locally built/changed jtreg will end up running against the jtreg installation pointed to by that environment variable. That effectively means that the changes being tested aren't tested against the actual changed code in jtreg and the test results can be misleading.

The commit in this PR generates a `make.sh` which resets the `JT_HOME` and `JTREG_HOME` environment variables to be empty so that the `jtreg` script that's launched from the locally built jtreg image will then pick up the correct locally built jtreg image.

I have run the jtreg self tests locally with this change and they continue to pass. Without this change, some of the tests fail because they end up picking a jtreg installation which doesn't have some recent changes that are available in this git repo (that's what prompted me to investigate this issue).

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Commit messages:
 - 7903692: JTREG_HOME and JT_HOME can interfere with jtreg self tests

Changes: https://git.openjdk.org/jtreg/pull/189/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=189&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903692
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/189.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/189/head:pull/189

PR: https://git.openjdk.org/jtreg/pull/189