tests: Add an implementation of the Logger interface for use gtests.

On exiting log types (panic and fatal), the message is set to an
ADD_FAILURE_AT macro, and the test is exited by throwing an otherwise
unexpected exception. On non-exiting log types, the message is sent to
the SUCCEEDED macro which currently doesn't output anything.

Change-Id: I1bb569e6cb8308dbc4c3e04eea7a962bd2b1ddd8
Reviewed-on: https://gem5-review.googlesource.com/6264
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>