[Process] Fix comparisons against process exitcode

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Use identical comparison instead of equal. In case the exitcode has not been caught, `Process::isSuccessful` returns true instead of false. It should not happen, but it's much more clean like that.