tests: fix shellcheck warnings in plugins/flt.lttng-utils.debug-info/test_succeed

tests: fix shellcheck warnings in plugins/flt.lttng-utils.debug-info/test_succeed

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

    In test_succeed line 38:
    this_dir_build="$BT_TESTS_BUILDDIR/$this_dir_relative"
    ^------------^ SC2034: this_dir_build appears unused. Verify use (or export if used externally).

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

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

    In test_succeed line 72:
            local expected_stdout=$(mktemp -t test_debug_info_stdout_expected.XXXXXX)
                  ^-------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_succeed line 73:
            local expected_stderr=$(mktemp -t test_debug_info_stderr_expected.XXXXXX)
                  ^-------------^ SC2155: Declare and assign separately to avoid masking return values.

Change-Id: Id9f0d3be21ef215a416946b702f915d98fb539d2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reported-by: shellcheck
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3136
Tested-by: jenkins <jenkins@lttng.org>