tests: fix shellcheck warnings in plugins/src.ctf.fs/succeed/test_succeed

tests: fix shellcheck warnings in plugins/src.ctf.fs/succeed/test_succeed

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

    In test_succeed line 69:
            local temp_stdout_output_file="$(mktemp -t actual_stdout.XXXXXX)"
                  ^---------------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_succeed line 70:
            local temp_greped_stdout_output_file="$(mktemp -t greped_stdout.XXXXXX)"
                  ^----------------------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_succeed line 71:
            local temp_stderr_output_file="$(mktemp -t actual_stderr.XXXXXX)"
                  ^---------------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_succeed line 103:
            local temp_stdout_output_file="$(mktemp -t actual_stdout.XXXXXX)"
                  ^---------------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_succeed line 104:
            local temp_stderr_output_file="$(mktemp -t actual_stderr.XXXXXX)"
                  ^---------------------^ SC2155: Declare and assign separately to avoid masking return values.

Change-Id: Ia67e0726aa898df7a761a3284fdd2919ac2570e8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reported-by: shellcheck
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3137
Tested-by: jenkins <jenkins@lttng.org>