[lldb][test] Re-enable TestRerunAndExprDylib.py

the `skipTestIfFn` requires a function that return a string to skip or None to run the test. The `isUbuntu18_04` function returns a bool and the test is skipped on all platforms.

Author: da-viper

lldb/test/API/functionalities/rerun_and_expr_dylib/TestRerunAndExprDylib.py

@@ -20,9 +20,9 @@ def isUbuntu18_04():
             with open(path) as f:
                 contents = f.read()
             if "Ubuntu 18.04" in contents:
-                return True
+                return "Ubuntu 18.04 is not supported."
 
-    return False
+    return None
 
 
 class TestRerunExprDylib(TestBase):