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>
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index 58563bd..13c4990 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -860,6 +860,7 @@
 
   call StopVimInTerminal(buf)
   call delete('Xtestfile.txt')
+  call delete('.Xtestfile.txt.un~')
 endfunc
 
 func Test_undo_range_normal()