[Process] Fix ExecutableFinder with open basedir

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This fixes the `ExecutableFinder` object to properly fetch the `open_basedir` setting, also added a bunch of tests for the `find()` method.
It's not possible to remove `open_basedir` restrictions at runtime, hence I permit `/`.