Tests: store debug logs on error

Rather than using the FakeLogger fixture, which always attaches
the log stream as a detail, use standard loggers that output to
a stringio, and then, only if a test fails, attach the full
log as a detail.

This allows us to report full debug-level logs for failing tests
in the gate (which normally has a limit on how large subunit files
can be).

Change-Id: I9e6509b7b69838d29582b040ef22f1d66010d45e
12 files changed