Use a different way to test warning emission

The test case was added in d08ef68
to verify that IRB emits Ruby warning as expected.

But the subject it uses relies on CRuby's regexp engine, which isn't always
used in other language implementations, like TruffleRuby. That's why we
ended up skipping TruffleRuby in this test case.

Since the test isn't about regexp itself, we can change the testing subject
and just remove the special condition for TruffleRuby.

Co-authored-by: Kevin Menard <kevin@nirvdrum.com>

test/irb/test_context.rb

@@ -57,10 +57,9 @@ def test_evaluate_with_encoding_error_without_lineno
       }
     end
 
-    def test_evaluate_with_onigmo_warning
-      pend if RUBY_ENGINE == 'truffleruby'
-      assert_warning("(irb):1: warning: character class has duplicated range: /[aa]/\n") do
-        @context.evaluate('/[aa]/', 1)
+    def test_evaluate_still_emits_warning
+      assert_warning("(irb):1: warning: END in method; use at_exit\n") do
+        @context.evaluate(%q[def foo; END {}; end], 1)
       end
     end