Only compare &nil syntax on rubies that support it

Author: Earlopain

test/prism/ruby/parameters_signature_test.rb

@@ -51,7 +51,7 @@ def test_nokey
     end
 
     def test_noblock
-      assert_parameters([[:noblock]], "&nil")
+      assert_parameters([[:noblock]], "&nil", compare: RUBY_VERSION >= "4.1")
     end
 
     def test_keyrest_anonymous
@@ -78,7 +78,7 @@ def test_forwarding
 
     private
 
-    def assert_parameters(expected, source)
+    def assert_parameters(expected, source, compare: true)
       # Compare against our expectation.
       assert_equal(expected, signature(source))