Added test case for mutables to libbe/settings_object.
This continues the line of changes started in libbe/properties with
the last two commits. Also straightened up stranch double-default in
libbe.settings_object.versioned_property and moved the fn_checked
before checked, which shouldn't matter because I never use both at
once, and can't think of a case where you'd want to.
I've also added some docstrings to the settings_object unit tests,
since apparently docstrings get printed during the test if they exist,
and they look nicer than the name of the unittest itself. More like
./configure output ;).