Add regression test #18501

ghc/ghc!3220 ended up fixing #18501. This patch adds a regression
test for #18501 to ensure that it stays fixed.

testsuite/tests/ghci/scripts/T18501.script

+:set -XAllowAmbiguousTypes
+class Foo a where foo :: String
+:t foo

testsuite/tests/ghci/scripts/T18501.stdout

+foo :: Foo a => String

testsuite/tests/ghci/scripts/all.T

@@ -318,4 +318,5 @@ test('T17403', normal, ghci_script, ['T17403.script'])
 test('T17431', normal, ghci_script, ['T17431.script'])
 test('T17549', normal, ghci_script, ['T17549.script'])
 test('T17669', [extra_run_opts('-fexternal-interpreter -fobject-code'), expect_broken(17669)], ghci_script, ['T17669.script'])
+test('T18501', normal, ghci_script, ['T18501.script'])
 test('T18644', normal, ghci_script, ['T18644.script'])