Wait on a breakpoint before checking output.
Add temp workaround for #1574: Flask tests fail with "no such option: --wait" on windows py27
Pin Flask version to last known good one in tests/requirements.txt.
Other fixes and improvements:
Make start_method mandatory for debug.Session to avoid problems with tests forgetting to specify it.
Add debug_me.scratchpad to enable async communication between test code and debuggee.
Improve debug.Session logging.
Improve test_attach.
* Replace wait_for_thread_stopped with wait_for_stop
* Switch flask tests to use flask server
* Assert linter
* Use marked lines
* Fix stack logging in tests
* Prevent black from formating some debug code
* Simplify flask tests
* Replace send_request(continue) with send_continue
* fix typo
