Fix exit code test for unix platform

UNIX adds 128 to signal codes.

Author: zygzagZ

test/support/acceptance_test.rb

@@ -755,7 +755,7 @@ class MyEngine < Rails::Engine
 
         artifacts = app.run("bin/rails runner 'system(\"kill -7 \#{Process.pid}\")'")
         code = artifacts[:status].exitstatus || artifacts[:status].termsig
-        assert_equal 7, code, "Expected exit status to be 7, but was #{code}"
+        assert_equal 7, code % 128, "Expected exit status to be 7, but was #{code}"
       end
     end
   end