Allow using coordinator and exporter programatically

[alextercete]

Description
We spin up a local coordinator and exporter for automated testing, and we want to be able to tell whether they're ready before running our tests. Also, we use port 0 with the coordinator to let the kernel allocate a free port, so we need to know which port was actually used. So now you can do:

labgrid-coordinator --listen 127.0.0.1:0

And it will print to stdout:

listening on 127.0.0.1:46269

We also fixed an issue when interrupting the coordinator/exporter (e.g. Ctrl+c):

^CINFO:root:Starting graceful shutdown...
Traceback (most recent call last):
  File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "path/to/labgrid/labgrid/remote/coordinator.py", line 1211, in <module>
    main()
  File "path/to/labgrid/labgrid/remote/coordinator.py", line 1204, in main
    loop.run_until_complete(serve(args.listen, cleanup))
  File "/usr/lib/python3.10/asyncio/base_events.py", line 636, in run_until_complete
    self.run_forever()
  File "/usr/lib/python3.10/asyncio/base_events.py", line 603, in run_forever
    self._run_once()
  File "/usr/lib/python3.10/asyncio/base_events.py", line 1871, in _run_once
    event_list = self._selector.select(timeout)
  File "/usr/lib/python3.10/selectors.py", line 469, in select
    fd_event_list = self._selector.poll(timeout, max_ev)
KeyboardInterrupt

Checklist

• PR has been tested

[codecov]

Codecov Report

❌ Patch coverage is 0% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.0%. Comparing base (ec5ec29) to head (ef785b9).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/remote/exporter.py	0.0%	15 Missing ⚠️
labgrid/remote/coordinator.py	0.0%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1867     +/-   ##
========================================
+ Coverage    45.8%   46.0%   +0.2%     
========================================
  Files         182     180      -2     
  Lines       14720   14462    -258     
========================================
- Hits         6743    6654     -89     
+ Misses       7977    7808    -169     

Flag	Coverage Δ
3.10	46.0% <0.0%> (+0.2%)	⬆️
3.11	46.0% <0.0%> (+0.2%)	⬆️
3.12	46.0% <0.0%> (+0.2%)	⬆️
3.13	45.9% <0.0%> (+0.2%)	⬆️
3.14	45.9% <0.0%> (+0.2%)	⬆️

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