introduce DEFAULT_AUTHOR constant
fix retrieval of email from ~/.oscrc

The Python rewrite broke this for the case where
~/.oscrc doesn't have a section for https://api.opensuse.org.
In normal usage this would never happen, but it occurs during
the test framework.
prevent tests depending on the contents of $HOME

Previously the tests would only pass if ~/.oscrc contained
a section for https://api.opensuse.org.  Since that is now
fixed, we can safely prevent any accidental dependency of
future tests on the contents of a real-world $HOME.