tests: make functions static in test_session.c

tests: make functions static in test_session.c

Fixes:

      CC       test_session.o
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:219:6: error: no previous declaration for ‘test_session_list’ [-Werror=missing-declarations]
     void test_session_list(void)
          ^~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:225:6: error: no previous declaration for ‘test_create_one_session’ [-Werror=missing-declarations]
     void test_create_one_session(void)
          ^~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:232:6: error: no previous declaration for ‘test_validate_session’ [-Werror=missing-declarations]
     void test_validate_session(void)
          ^~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:258:6: error: no previous declaration for ‘test_destroy_session’ [-Werror=missing-declarations]
     void test_destroy_session(void)
          ^~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:278:6: error: no previous declaration for ‘test_duplicate_session’ [-Werror=missing-declarations]
     void test_duplicate_session(void)
          ^~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:284:6: error: no previous declaration for ‘test_session_name_generation’ [-Werror=missing-declarations]
     void test_session_name_generation(void)
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/lttng-tools/tests/unit/test_session.c:309:6: error: no previous declaration for ‘test_large_session_number’ [-Werror=missing-declarations]
     void test_large_session_number(void)
          ^~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ib1f6a004e6feb4693c4331a8b0ce67ee1697703c
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>