[lldb] re-add skipIfOutOfTreeDebugserver for TestRegisters

The swiftlang CI bots are running a pretty old Xcode install
that doesn't have the x86 debugserver change that tries to
fetch additional registers -- and this test which confirms that
they are unavailable is not passing.  If the CI bot debugserver
is ever updated, this can be removed again.

Author: jasonmolenda

lldb/test/API/commands/register/register/register_command/TestRegisters.py

@@ -41,6 +41,7 @@ def get_sme_available(self):
 
     @skipIfiOSSimulator
     @skipIf(archs=no_match(["amd64", "arm$", "i386", "x86_64"]))
+    @skipIfOutOfTreeDebugserver
     @expectedFailureAll(oslist=["freebsd", "netbsd"], bugnumber="llvm.org/pr48371")
     def test_register_commands(self):
         """Test commands related to registers, in particular vector registers."""