conftest: Fix log level settings for stdout

conftest: Fix log level settings for stdout

This patch fixes bug #272 ("conftest ignores log settings for stdout").
http://wiki.strongswan.org/issues/272

According to the documentation of add_logger in src/libcharon/bus/bus.h,
the relevant log levels of a logger are registered with the logging
subsystem when adding the logger. If the log levels change later, the
logger must be re-added to propagate the new settings. In conftest.c,
the stdout logger is initialized and added before reading the logging
settings, but wasn't re-added after reading the settings.