http-service tests: Speed up execution by re-using the same http-service instance

Reduces `make check-pytest` run time from 40s to 33s on my machine.  This
change is also good because `make check-pytest-boxed` will additionally
run the tests with the fixtures starting up and being torn down each time
so this should increase test-coverage.
lirc: Change lirc config and improve lirc tests

The `run-test-job` config for lirc used to look like:

    'lirc_from_test_pack': {
        'config_filename': 'xyz.lircd.conf',
        'driver': 'devinput',
        'device': '/dev/lirc0'
    }

Now it looks like:

    'lirc': {
        'config': {'from_test_pack': 'xyz.lircd.conf'},
        'driver': 'devinput',
        'device': '/dev/lirc0'
    }

This is better because it allows us to change the source of the config
easily without messing with the driver/device settings that will be true
for all `lircd` processes.

This also introduces a fake lircd which is used in tests to check that the
right keys are being pressed.
tests: Tidy up unnecessary copy-and-pasted code
Tests: Improve logging of `test_that_line_number_is_reported_during_test_run`

The `job.log`s show us the `stbt batch` and `lirc` output.
Add `/v1/device/press` endpoint

This endpoint allows the user to send key-presses when the system is
not running tests.  Currently we bring lirc up and tear it back down
again for every keypress.  This keeps things simple as we don't have
to worry about working out when to start and stop `lircd` and whether
we need to restart due to a configuration change, but is obviously
inefficient.  Depending on how well this works in practice we may
have to replace this implementation.
Merge branch 'press-endpoint'

* press-endpoint:
  Add `/v1/device/press` endpoint
  Tests: Improve logging of `test_that_line_number_is_reported_during_test_run`
  tests: Tidy up unnecessary copy-and-pasted code
  lirc: Change lirc config and improve lirc tests
  http-service tests: Speed up execution by re-using the same http-service instance