(PUP-11897) Add additional mocking to the execution spec

With the change in 4f35fd2, the execution tests need to have
mocked out the an addtional call to `waitpid2` that has the
flag set to zero.

spec/unit/util/execution_spec.rb

@@ -29,6 +29,7 @@ def stub_process_wait(exitstatus)
         allow(FFI::WIN32).to receive(:CloseHandle).with(thread_handle)
       else
         allow(Process).to receive(:waitpid2).with(pid, Process::WNOHANG).and_return(nil, [pid, double('child_status', :exitstatus => exitstatus)])
+        allow(Process).to receive(:waitpid2).with(pid, 0).and_return(nil, [pid, double('child_status', :exitstatus => exitstatus)])
         allow(Process).to receive(:waitpid2).with(pid).and_return([pid, double('child_status', :exitstatus => exitstatus)])
       end
     end