Run different selftests when debug mode is engaged

Add two skiplist tuples to the selftest framework, one for tests to
remove when debug mode is on, another for tests to remove when debug
mode is off. This is not used yet, so the tuples are initially
empty. Also enable debug mode in the daemon config fixture so that
debug functionality can be tested in the normal selftest run. The
default example config (tested with tox -e selftest_config) covers
the debug disabled case.