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_shortpathname.vim b/src/testdir/test_shortpathname.vim
index 6b3f85f..3c09b4c 100644
--- a/src/testdir/test_shortpathname.vim
+++ b/src/testdir/test_shortpathname.vim
@@ -51,7 +51,7 @@
   call TestIt(file2, ':p:8', resfile2)
   call TestIt(nofile2, ':p:8', resnofile2)
   call TestIt(nofile2, ':p:8:h', fnamemodify(resnofile2, ':h'))
-  exe 'cd ' . dir1
+  call chdir(dir1)
   call TestIt(file1, ':.:8', strpart(resfile1, strlen(resdir1)+1))
   call TestIt(nofile1, ':.:8', strpart(resnofile1, strlen(resdir1)+1))
   call TestIt(file2, ':.:8', strpart(resfile2, strlen(resdir1)+1))
@@ -68,7 +68,7 @@
   call delete(dir2, 'd')
   call delete(dir1, 'd')
 
-  exe "cd " . save_dir
+  call chdir(save_dir)
 endfunc
 
 func Test_ColonEight_MultiByte()