The python module is now pulled in autoload/UltiSnips.vim. This means
that parsing of the .vimrc will only map the keys and set some options -
very cheap.
Unfortunately, the autocommands set up in plugin/UltiSnips.vim pulls in
the python code basically immediately still.
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.
