lisa.tests.base: Add TestConfBase class
lisa.tests.base: Add TestConfBase class

Allow hosting all the configurations under a single "test-conf"
top-level key.

BREAKING CHANGE

The DmesgTestConf class top-level key is now test-conf/dmesg instead of
dmesg-test-conf.

credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa