Exclude problematic test on TruffleRuby

Author: eregon

spec/concurrent/executor/safe_task_executor_spec.rb

@@ -110,17 +110,20 @@ def execute
 
         it 'should return success' do
           (pending('possible local jump bug on JRuby https://github.com/jruby/jruby/issues/7136'); fails) if Concurrent.on_jruby?
+          skip('the test does not make sense: https://github.com/ruby-concurrency/concurrent-ruby/issues/931') if Concurrent.on_truffleruby?
           success, _value, _reason = subject
           expect(success).to be_truthy
         end
 
         it 'should return a nil value' do
+          skip('the test does not make sense: https://github.com/ruby-concurrency/concurrent-ruby/issues/931') if Concurrent.on_truffleruby?
           _success, value, _reason = subject
           expect(value).to be_nil
         end
 
         it 'should return a nil reason' do
           (pending('possible local jump bug on JRuby https://github.com/jruby/jruby/issues/7136'); fails) if Concurrent.on_jruby?
+          skip('the test does not make sense: https://github.com/ruby-concurrency/concurrent-ruby/issues/931') if Concurrent.on_truffleruby?
           _success, _value, reason = subject
           expect(reason).to be_nil
         end