Tests/Kernel: Reset SIGCHLD's handler at the end of TestWaitFirstNoHang

With this change, when we're done with our test the SIGCHLD handler is
set back to the default one. This fixes an issue when running the test
under LSan with libsanitizers. LSan works by forking and then using
ptrace on the parent to inspect its address space, when this child exits
it was triggering our handler and caused a crash. Restoring the default
handler solves the issue. For some reason, this only happens with GCC's
implementation of LSan while this works fine with Clang's.

Tests/Kernel/POSIX/Wait/TestWaitFirstNoHang.cpp

@@ -23,11 +23,11 @@ static void sigchld_handler(int, siginfo_t* siginfo, void*)
 
 static void run_test(auto callback)
 {
-    struct sigaction s;
+    struct sigaction s, old_sigaction;
     s.sa_sigaction = sigchld_handler;
     sigemptyset(&s.sa_mask);
     s.sa_flags = SA_SIGINFO;
-    TRY_OR_FAIL(Core::System::sigaction(SIGCHLD, &s, nullptr));
+    TRY_OR_FAIL(Core::System::sigaction(SIGCHLD, &s, &old_sigaction));
 
     g_wait_caller = move(callback);
 
@@ -46,6 +46,8 @@ static void run_test(auto callback)
 
     sigsuspend(&prev);
     EXPECT_EQ(sigprocmask(SIG_SETMASK, &prev, nullptr), 0);
+
+    TRY_OR_FAIL(Core::System::sigaction(SIGCHLD, &old_sigaction, nullptr));
 }
 
 TEST_CASE(waitid)