Add utility logging functions to isctest.log

Add utility logging functions to isctest.log

Unify the different loggers (conftest, module, test) into a single
interface. Remove the need to select the proper logger by automatically
selecting the most-specific logger currently available.

This also removes the need to use the logger/mlogger fixtures manually
and pass these around. This was especially annoying and unwieldy when
splitting the test cases into functions, because logger had to always be
passed around. Instead, it is now possible to use the
isctest.log.(debug,info,warning,error) functions.