Reset the unnamed register at the start of a test.

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.

test/vim_test_case.py

@@ -357,6 +357,7 @@ class VimTestCase(unittest.TestCase, TempFileManager):
         vim_config.append('set fileencoding=utf-8')
         vim_config.append('set buftype=nofile')
         vim_config.append('set shortmess=at')
+        vim_config.append('let @" = ""')
         vim_config.append('let g:UltiSnipsExpandTrigger="<tab>"')
         vim_config.append('let g:UltiSnipsJumpForwardTrigger="?"')
         vim_config.append('let g:UltiSnipsJumpBackwardTrigger="+"')