tests: make functions static in test_utils_expand_path.c

Fixes:

      CC       test_utils_expand_path.o
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_expand_path.c:126:5: error: no previous declaration for ‘prepare_valid_results’ [-Werror=missing-declarations]
     int prepare_valid_results(void)
         ^~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_expand_path.c:182:5: error: no previous declaration for ‘free_valid_results’ [-Werror=missing-declarations]
     int free_valid_results(void)
         ^~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_expand_path.c:195:5: error: no previous declaration for ‘prepare_symlink_tree’ [-Werror=missing-declarations]
     int prepare_symlink_tree(void)
         ^~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_utils_expand_path.c:235:5: error: no previous declaration for ‘free_symlink_tree’ [-Werror=missing-declarations]
     int free_symlink_tree(void)
         ^~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I220a2f8dadb355adb4fb974a2f3a0e161b0618e6
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>