[lldb] Skip instead of XFAIL TestGdbRemote_vContThreads on Darwin

The two XFAILed tests started timing out after D126983. Given that they
were XFAILed anyway I didn't investigate and just skipped them.

Author: JDevlieghere

lldb/test/API/tools/lldb-server/vCont-threads/TestGdbRemote_vContThreads.py

@@ -54,8 +54,8 @@ def get_pid(self):
         return int(procinfo['pid'], 16)
 
     @skipIfWindows
+    @skipIfDarwin
     @expectedFailureNetBSD
-    @expectedFailureDarwin # No signals delivered
     @expectedFailureAll(oslist=["freebsd"],
                         bugnumber="github.com/llvm/llvm-project/issues/56086")
     @skipIfAsan # Times out under asan
@@ -84,10 +84,10 @@ def test_signal_one_thread(self):
             threads[:1])
 
     @skipIfWindows
+    @skipIfDarwin
     @expectedFailureNetBSD
     @expectedFailureAll(oslist=["freebsd"],
                         bugnumber="github.com/llvm/llvm-project/issues/56086")
-    @expectedFailureDarwin # Only one signal delivered
     @skipIfAsan # Times out under asan
     @skipIf(oslist=["linux"], archs=["arm", "aarch64"]) # Randomly fails on buildbot
     def test_signal_all_threads(self):