tests: add and use bt_grep_ok

When a test that consists of matching a pattern in a file using grep
fails (sometimes just in the CI), it would be useful to see the file
contents to understand what happened.  Add the `bt_grep_ok` utility
function.  It tries to match a pattern in a file using `grep`, then
issues a test result.  If the test failed, it prints the content of the
file on stderr.

Change-Id: I464e414b334052677799ce201483095fa9bff4c3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11169
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>