test-nointerrupt: make "sure" the handler "might" trigger (issue6558) We are sure that the signal got sent in the right time frame, however, we still have race, so either the code is actually buggy or we need some security to make sure the signal get processed. We might be affected by https://bugs.python.org/issue43406 ? Differential Revision: https://phab.mercurial-scm.org/D11251

testing: make sure write_file is "atomic" This make sure viewer cannot see the new file with partial content. This was likely the cause of some flakiness in `test-nointerrupt.t` Differential Revision: https://phab.mercurial-scm.org/D11250