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Also adjust affected tests accordingly.

This makes some tests pass which uses logger_example, which is
a standalone binary.

 * Also set this env variable in isc::log::initLogger()

The change to make DEBUG the default logging severity generates a
lot of output.  C++ unit test output is now routed to /dev/null by
default, with B10_LOGGER_DESTINATION needing to be set to stdout
or stderr to make it visible.

Add support of B10_LOGGER_DESTINATION to choose logging destination
for unit tests, and add tests to check that they work.  Also update
some documentation.