[Process] Fixes to executable finder tests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

A couple of fixes for ExecutableFinder tests:
- Use PHPUnit ini_set wrapper to avoid side effects.
- Skip a test on Windows (similar to an earlier test that was already skipped).
- Removed an unused variable.