Tests: Rename helper functions to have consistent names

Most of the helper functions had inconsistent naming.

Suggested naming convention for helper functions:

<action>_<bin_name>
	or
<action>_lttng_<command>

List of modified helper functions:

lttng_start_relayd -> start_lttng_relayd
lttng_stop_relayd  -> stop_lttng_relayd

start_sessiond     -> start_lttng_sessiond
stop_sessiond      -> stop_lttng_sessiond

start_tracing      -> start_lttng_tracing
stop_tracing       -> stop_lttng_tracing

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>