Add a capture_logging test util

[Claude Paroz]

While working on silencing contrib.gis tests (to-be-created ticket), I find myself in need of a utility to capture logging during tests. This is the result this work.

[Claude Paroz]

Add context manager for logging capture

[Claude Paroz]

to-be-created ticket is #17959 (which represents a real use case for this ticket).

[Preston Holmes]

#19866 added a similar utility, but differs from this patch in several ways, one it was added as private API, not documented for use in other's tests. Second it took the approach close to mocking the logger via monkey-patch of a specific level method, rather than adding a stream to the logging machinery.

This should stay open as it has a different scope of packaging this sort of utility for public use - not something I feel strongly that Django needs to ship publicly, but not something I'm against either.

[Tim Graham]

We resolved the linked ticket without this -- do we still want/need it?

In any case, I'm sure it needs to be updated to apply cleanly.

[Claude Paroz]

Let's forget it for now. We'll see in the future if there is still some demand for it.