test: Avoid failing tests on unknown kill_server behavior

[rmccorm4]

CLI tests in CI intermittently fail with AttributeError: ScopedTritonServer has no attribute self.proc. I haven't dug deeper to see why this is happening yet - but this adds an initialization and some error handling to try to avoid failing tests on this behavior.

    def kill_server(self, timeout: int = 60):
        try:
>           self.proc.terminate()
E           AttributeError: 'ScopedTritonServer' object has no attribute 'proc'
tests/utils.py:151: AttributeError