Fix btest in ractor_test.rb that can lead timeout of the test

It could also potentially lead to an out of memory error.

Author: luke-gruber

bootstraptest/test_ractor.rb

@@ -1455,11 +1455,19 @@ class C
       loop do
         10_000.times.map { Object.new }
         port << Time.now
+        Ractor.receive
       end
     end
   end
 
-  1_000.times { port.receive }
+  100.times {
+    workers.each do
+      port.receive
+    end
+    workers.each do |w|
+      w.send(nil)
+    end
+  }
   "ok"
 } if !yjit_enabled? && ENV['GITHUB_WORKFLOW'] != 'ModGC' # flaky