Improve encoding error test case

The test input IRB currently uses happen to hit a compatibility bug in
TruffleRuby, which has been documented in
oracle/truffleruby#2848

Although it'll eventually be fixed, we can make the test case support TruffleRuby
now by tweaking it just a little bit.

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

test/irb/test_context.rb

@@ -50,9 +50,9 @@ def test_evaluate_with_exception
     end
 
     def test_evaluate_with_encoding_error_without_lineno
-      pend if RUBY_ENGINE == 'truffleruby'
+      # TruffleRuby adds 'encoding' in the middle of the message
       assert_raise_with_message(EncodingError, /invalid symbol/) {
-        @context.evaluate(%q[{"\xAE": 1}], 1)
+        @context.evaluate(%q[:"\xAE"], 1)
         # The backtrace of this invalid encoding hash doesn't contain lineno.
       }
     end