Fix potential deadlock in TestMdns

[andy31415]

Problem

if testmdns fails (e.g. avahi not running), the test will deadlock forever:

• wait_for requires that lock is aquired when exiting
• lock was set for the entire 'run main loop' which never finishes

Summary of Changes

Narrow down the scope of the loops.
Add a start and done condition as separate tasks.
Attempt to cleanly shut down the chip stack on error (this does NOT work, however I was unable to debug as to why ... this test has logic flaws in terms of how the chip main loop is handled)

[todo]

the above does not seem to actually reliably shut down the chip

connectedhomeip/src/platform/tests/TestMdns.cpp

Lines 140 to 150 in a88d8ee

	// TODO: the above does not seem to actually reliably shut down the chip
	// Program will abort with core because chip thread will still run.
	doneCondition.wait_for(lock, std::chrono::seconds(1));
	if (!done)
	{
	fprintf(stderr, "Orderly shutdown of the platform main loop failed as well.\n");
	}
	retVal = EXIT_FAILURE;
	}
	}
	return retVal;

---

This comment was generated by todo based on a TODO comment in a88d8ee in #6721. cc @andy31415.

[todo]

the above does not seem to actually reliably shut down the chip stack.

connectedhomeip/src/platform/tests/TestMdns.cpp

Lines 140 to 150 in 5724b4b

	// TODO: the above does not seem to actually reliably shut down the chip stack.
	// Program will abort with core because chip thread will still run.
	doneCondition.wait_for(lock, std::chrono::seconds(1));
	if (!done)
	{
	fprintf(stderr, "Orderly shutdown of the platform main loop failed as well.\n");
	}
	retVal = EXIT_FAILURE;
	}
	}
	return retVal;

---

This comment was generated by todo based on a TODO comment in 5724b4b in #6721. cc @andy31415.

[woody-apple]

@saurabhst?