Add a test for #21765

This issue (of reporting a constraint as being redundant even though
removing it causes typechecking to fail) was fixed in aed1974e.
This commit simply adds a regression test.

Fixes #21765

testsuite/tests/typecheck/should_compile/T21765.hs

+{-# LANGUAGE UndecidableInstances, FlexibleInstances #-}
+
+{-# OPTIONS_GHC -Wredundant-constraints #-}
+
+module T21765 where
+
+class Functor f => C f where c :: f Int
+
+instance (Functor f, Applicative f) => C f where c = pure 42

testsuite/tests/typecheck/should_compile/all.T

@@ -847,6 +847,7 @@ test('DeepSubsumption06', normal, compile, ['-XHaskell98'])
 test('DeepSubsumption07', normal, compile, ['-XHaskell2010'])
 test('DeepSubsumption08', normal, compile, [''])
 test('DeepSubsumption09', normal, compile, [''])
+test('T21765', normal, compile, [''])
 test('T21951a', normal, compile, ['-Wredundant-strictness-flags'])
 test('T21951b', normal, compile, ['-Wredundant-strictness-flags'])
 test('T21550', normal, compile, [''])