Revert "Merge pull request #5316 from adrian-prantl/100034078"

This reverts commit 76d733c, reversing
changes made to b286431.

Author: kastiglione

lldb/test/API/lang/swift/unknown_reference/TestSwiftUnknownReference.py

@@ -27,7 +27,6 @@ def check_class(self, var_self):
         lldbutil.check_variable(self, m_string, summary='"world"')
 
     @swiftTest
-    @skipIfDarwin # This test fails non-reproducibly in CI. rdar://100034078
     @skipUnlessFoundation
     def test_unknown_objc_ref(self):
         """Test unknown references to Objective-C objects."""