patch 8.1.2278: using "cd" with "exe" may fail

Problem:    Using "cd" with "exe" may fail.
Solution:   Use chdir() instead.
diff --git a/src/testdir/test_expand.vim b/src/testdir/test_expand.vim
index 53f753d..48dce25 100644
--- a/src/testdir/test_expand.vim
+++ b/src/testdir/test_expand.vim
@@ -35,7 +35,7 @@
   call assert_true(isdirectory('Xdir ~ dir'))
   cd Xdir\ ~\ dir
   call assert_true(getcwd() =~ 'Xdir \~ dir')
-  exe 'cd ' . fnameescape(dir)
+  call chdir(dir)
   call delete('Xdir ~ dir', 'd')
   call assert_false(isdirectory('Xdir ~ dir'))
 endfunc