tests: add and use bt_grep

Add the `bt_grep` function, which passes all its arguments to the grep
binary specified by BT_TESTS_GREP_BIN.  Use everywhere where applicable
in the testsuite.

Change-Id: Ia166106fd85312e681fa2692e1f5409c9556f897
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11248
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
CI-Build: Michael Jeanson <mjeanson@efficios.com>