Tests: Pull _fake_lircd into seperate function
Tests: Pull _fake_lircd into seperate function

So we can use it in a subsequent test.  We also:

* Fix a potential race condition by removing a `sleep`
* Make `_fake_lircd` generally more robust to changes in stbt's
  implementation
* Fix a potential race condition with use of `tmpname()`
* Avoid polluting `/tmp` with old no-longer-listening sockets
* Make the `test_that_lirc_remote_is_symmetric_with_lirc_remote_listen`
  test more readable.
* Use multiprocessing rather than thread for improved robustness

martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester