patch 9.0.2008: test: undofile left behind

patch 9.0.2008: test: undofile left behind

Problem:  test: undofile left behind
Solution: cleanup undofile

fix: tmp file not deleted when running make test_undo

Temporary file `.Xtestfile.txt.un~` was left running `make test_undo`
and vim was configured with:
```
./configure --with-features=normal --enable-gui=no --enable-terminal
```

closes: #13304

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Dominique Pellé <dominique.pelle@tomtom.com>