tests: tools/clear/test_ust wait for specific test app pid

tests: tools/clear/test_ust wait for specific test app pid

Observed issue
==============

When debugging failing tests manually, one step that is sometimes done
is to quickly swap the commands that start the relay or sessiond in
`tests/utils/utils.sh` (eg. in `start_lttng_relayd_opt`) for the version
which uses a verbose output to a logfile.

When doing this, the `relayd` wasn't using the background
`process_mode`, and was a child of the running test.

This caused `test_ust_local_snapshot_per_pid` in
`tests/regression/tools/clear/test_ust` to hang as it waited for all
child processes to terminate.

Solution
========

The test has been updated to wait for only the specific test application
pid.

Known drawbacks
===============

None.

Change-Id: I8761649a52fceda92a5545c71818dc2eb027bfcf
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>