build: Enable automake parallel-tests

This should speed up the test runs somewhat, as well as improving log
output handling.

https://bugzilla.gnome.org/show_bug.cgi?id=703192
tests: Remove debug log storage system

automake's parallel-tests feature now automatically separates and stores
logs, and only prints them out on test failure. This is exactly what the
logging code in the tests was doing before, so the logging code can now
be simplified.

https://bugzilla.gnome.org/show_bug.cgi?id=703192