Close #469 Add GStreamer state tests

Add 3 test cases to test GStreamer states: record, pause, and stop

test_current_state_is_record(): checks that the current state is RECORD
test_current_state_is_pause(): checks that the current state is PAUSED
test_current_state_is_not_stop(): sets state to null, then checks that
current state is not STOP

Related #437