tests: make functions static in test_notification.c

Fixes:

      CC       test_notification.o
    /home/smarchi/src/lttng-tools/tests/unit/test_notification.c:50:6: error: no previous declaration for ‘test_condition_buffer_usage’ [-Werror=missing-declarations]
     void test_condition_buffer_usage(struct lttng_condition *buffer_usage_condition)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_notification.c:265:6: error: no previous declaration for ‘test_condition_buffer_usage_low’ [-Werror=missing-declarations]
     void test_condition_buffer_usage_low(void)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_notification.c:280:6: error: no previous declaration for ‘test_condition_buffer_usage_high’ [-Werror=missing-declarations]
     void test_condition_buffer_usage_high(void)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_notification.c:295:6: error: no previous declaration for ‘test_action’ [-Werror=missing-declarations]
     void test_action(void)
          ^~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_notification.c:305:6: error: no previous declaration for ‘test_trigger’ [-Werror=missing-declarations]
     void test_trigger(void)
          ^~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I4df29ec42436798005e740d5b6fb34973ea22350
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>