discovery_client: Fix auto-launching discovery service producing a ResourceWarning

[vigkre]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

• Save the reference of the subprocess.popen object to a global variable.
• This prevents leakage of subprocess which in turn denotes that the subprocess is still running and produces a ResourceWarning.

Why should this Pull Request be merged?

• Fixes -- Auto-launching the discovery service leaks a subprocess.Popen, which produces a ResourceWarning.
• Issue Auto-launching discovery service produces a ResourceWarning #331

What testing has been done?

1. Ran example (nidaqmx_analog_input) service and make sure it is registered with the discovery service.
2. Used Task Manager to kill the discovery service.
3. Manually launched the example service with poetry run python measurement.py -v.
4. Verified ResourceWarning doesn't happen anymore.
5. Repeated step 1 to 4 for other examples as well.
6. For other examples, in step 3 used poetry run python -X dev measurement.py to reproduce the ResourceWarning.

[github-actions]

Test Results

     12 files  ±0       12 suites  ±0   18m 3s ⏱️ + 1m 7s
   180 tests ±0     180 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
2 148 runs  ±0  2 148 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 49a2a81. ± Comparison against base commit c89b733.

♻️ This comment has been updated with latest results.