tests: add valgrind suppression for test-general-with-expect

tests: add valgrind suppression for test-general-with-expect

    ==8781== Syscall param rt_sigaction(act->sa_flags) points to uninitialised byte(s)
    ==8781==    at 0x5E547DD: __libc_sigaction (sigaction.c:64)
    ==8781==    by 0x8149025: unref_unix_signal_handler_unlocked (gmain.c:4981)
    ==8781==    by 0x81490BF: g_child_watch_finalize (gmain.c:5025)
    ==8781==    by 0x8149535: g_source_unref_internal (gmain.c:1997)
    ==8781==    by 0x814C377: g_main_context_dispatch (gmain.c:3094)
    ==8781==    by 0x814C627: g_main_context_iterate.isra.24 (gmain.c:3713)
    ==8781==    by 0x814CA39: g_main_loop_run (gmain.c:3907)
    ==8781==    by 0x11AC8B: test_nm_utils_kill_child_async_do (test-general-with-expect.c:90)
    ==8781==    by 0x119769: test_nm_utils_kill_child (test-general-with-expect.c:292)
    ==8781==    by 0x81705E0: g_test_run_suite_internal (gtestutils.c:2067)
    ==8781==    by 0x81707A5: g_test_run_suite_internal (gtestutils.c:2138)
    ==8781==    by 0x8170B1A: g_test_run_suite (gtestutils.c:2189)
    ==8781==  Address 0xffeffed28 is on thread 1's stack
    ==8781==
    {
       <insert_a_suppression_name_here>
       Memcheck:Param
       rt_sigaction(act->sa_flags)
       fun:__libc_sigaction
       fun:unref_unix_signal_handler_unlocked
       fun:g_child_watch_finalize
       fun:g_source_unref_internal
       fun:g_main_context_dispatch
       fun:g_main_context_iterate.isra.24
       fun:g_main_loop_run
       fun:test_nm_utils_kill_child_async_do
       fun:test_nm_utils_kill_child
       fun:g_test_run_suite_internal
       fun:g_test_run_suite_internal
       fun:g_test_run_suite
    }