Fail unless a log message of level *level* or higher is emitted
on *logger_name* or its children. If omitted, *level* defaults to
INFO and *logger* defaults to the root logger.
This method must be used as a context manager, and will yield
a recording object with two attributes: `output` and `records`.
At the end of the context manager, the `output` attribute will
be a list of the matching formatted log messages and the
`records` attribute will be a list of the corresponding LogRecord
objects.
Optionally supply `formatter` to control how messages are formatted.
Example::
with self.assertLogs('foo', level='INFO') as cm:
logging.getLogger('foo').info('first message')
logging.getLogger('foo.bar').error('second message')
self.assertEqual(cm.output, ['INFO:foo:first message',
'ERROR:foo.bar:second message'])