Can now split recording with bitrate 0

Firmware 702 fixes the issue with bitrate 0 not producing SPS/PPS
headers in H.264 encoding. Removed the warning when bitrate was 0 and
adjusted tests to ensure it works (rather than ensuring it fails)
Remove tempdirs immediately after use

Running out of space on the SD card in the dev rig while testing; turns
out py.test only removes directories created by the tmpdir fixture after
several runs. This has now been replaced with a custom fixture that
removes temp dirs immediately after use.