tests: pass trace path prefix down in test_live

tests: pass trace path prefix down in test_live

A future test case will need to use a different trace path prefix than
the existing tests.  Add a parameter for that and pass the value down.

Change-Id: I2b95ce098461f32503cafabfc640d99fe120e582
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10919
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>