[CMAKE] Move the test log files to test/log

Summary:
This puts all the log files (generated by the wrapper) into a single
place, which makes them easier to find. They are also cleaned properly
by ninja clean.

Test Plan:
Ensure the log files are put into test/log:

ninja check

Ensure the test/log directory is removed:

ninja clean

Ensure the logs are in the artifact directory:

../contrib/teamcity/build-configurations.py build-without-wallet

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7222