Fix logcapture plugin to capture output from non-propagating loggers,…

[adamchainz]

… and to not output 'no handlers could be found' message.

Found this when activating log capturing on my project's test suite, we would end up with a single message in the test output such as No handlers could be found for logger "foo.yes" for a logger that has propagate = False. This is because the logcapture plugin would clear the handlers and not replace them.

logging emits this message once per program: https://hg.python.org/cpython/file/2.7/Lib/logging/__init__.py#l1339

To fix this, I install the memory handler for every non-propagating logger (have to take care to find only legitimate loggers with propagate = False and not e.g. Placeholder objects). This not only stops the message from appearing but also captures their output which may help further with debugging.

[jszakmeister]

Seems reasonable. Thank you.

[adamchainz]

🌈 🐰 🌈