Add test for #6091.

testsuite/tests/ghci/scripts/T6091.hs

+module T6091 where
+
+infixr 4 `foo`
+foo :: Int -> Int -> Int
+foo x y = x * y

testsuite/tests/ghci/scripts/T6091.script

+:l T6091.hs
+:info foo

testsuite/tests/ghci/scripts/T6091.stdout

+foo :: Int -> Int -> Int 	-- Defined at T6091.hs:5:1
+infixr 4 `foo`

testsuite/tests/ghci/scripts/all.T

@@ -126,3 +126,4 @@ test('T5975b',
 test('T6027ghci', normal, ghci_script, ['T6027ghci.script'])
 
 test('T6007', normal, ghci_script, ['T6007.script'])
+test('T6091', normal, ghci_script, ['T6091.script'])