Change singledispatch test to non-xfail

Change a singledispatch test that tested whether multiple underscore
functions being used wasn't an error from xfail to regular test, as the
previous commits made that not be an error any more.

Author: pranavrajpal

test-data/unit/check-singledispatch.test

@@ -16,7 +16,7 @@ fun(1) # E: Argument 1 to "fun" has incompatible type "int"; expected "A"
 # probably won't be required after singledispatch is special cased
 [builtins fixtures/args.pyi]
 
-[case testMultipleUnderscoreFunctionsIsntError-xfail]
+[case testMultipleUnderscoreFunctionsIsntError]
 from functools import singledispatch
 
 @singledispatch