[vim] correctly append to register and update the unnamed register to match last used register

Test for the following were added:
Yanking a 
- word then a line should result in two lines:  [word]\n[line]
- word then a word should result in one line:  [word][word]
- line then a line should result in two lines:  [line]\n[line]
- line then a word should result in two lines:  [line]\n[word]
The unnamed register should be identical to last used register in any case.