It's possible that when using _vim_dec with Python 2 that we fail to
convert the string to Unicode and just return the raw string instead.
This could happen, for instance, when there was arbitrary data in the
unnamed register during the save/restore process. During restoration,
we'd attempt to encode the string, but this may fail with a
UnicodeDecodeError as Python attempts to first convert to Unicode and
then to the requested encoding.
To fix this, let's also catch `UnicodeDecodeError` and return the raw
string if we fail to convert.
Any test that alters the unnamed register can cause subsequent tests to
fail if there is an issues saving and restoring the register. Let's
make the test infrastructure more robust against this issue by resetting
the unnamed register to an empty string.
