tests: fix shellcheck warnings in plugins/flt.utils.muxer/succeed/test_succeed

tests: fix shellcheck warnings in plugins/flt.utils.muxer/succeed/test_succeed

Fix the path to utils.sh, so that shellcheck finds it.  Then, fix:

    In test_succeed line 70:
            run_test $i
                     ^-- SC2086: Double quote to prevent globbing and word splitting.

    Did you mean:
            run_test "$i"

Change-Id: I76e1fdf5569c5acc487d0b55abcf1d9babd242cd
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reported-by: shellcheck
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3135
Tested-by: jenkins <jenkins@lttng.org>