Fix spec to correctly wait for the thread to be waiting in the require lock

Author: eregon

spec/ruby/fixtures/code/concurrent.rb

@@ -2,7 +2,7 @@
 Thread.current[:in_concurrent_rb] = true
 
 if t = Thread.current[:wait_for]
-  Thread.pass until t.backtrace && t.backtrace.any? { |call| call.include? 'require' }
+  Thread.pass until t.backtrace && t.backtrace.any? { |call| call.include? 'require' } && t.stop?
 end
 
 if Thread.current[:con_raise]