patch 9.0.0719: too many delete() calls in tests

Problem:    Too many delete() calls in tests.
Solution:   Use deferred delete where possible.
diff --git a/src/testdir/test_shortpathname.vim b/src/testdir/test_shortpathname.vim
index 79b7899..5964630 100644
--- a/src/testdir/test_shortpathname.vim
+++ b/src/testdir/test_shortpathname.vim
@@ -42,7 +42,7 @@
   let file2 = dir2 . '/z.txt'
   let nofile2 = dir2 . '/zz.txt'
 
-  call mkdir(dir1)
+  call mkdir(dir1, 'D')
   let resdir1 = substitute(fnamemodify(dir1, ':p:8'), '/$', '', '')
   call assert_match('\V\^c:/XX\x\x\x\x~1.Y\$', resdir1)
 
@@ -52,9 +52,9 @@
   let resfile2 = resdir2 . '/z.txt'
   let resnofile2 = resdir2 . '/zz.txt'
 
-  call mkdir(dir2)
-  call writefile([], file1)
-  call writefile([], file2)
+  call mkdir(dir2, 'D')
+  call writefile([], file1, 'D')
+  call writefile([], file2, 'D')
 
   call TestIt(file1, ':p:8', resfile1)
   call TestIt(nofile1, ':p:8', resnofile1)
@@ -73,10 +73,6 @@
   call TestIt(nofile2, ':~:8', '~' . strpart(resnofile2, strlen(resdir1)))
 
   cd c:/
-  call delete(file2)
-  call delete(file1)
-  call delete(dir2, 'd')
-  call delete(dir1, 'd')
 
   call chdir(save_dir)
 endfunc
@@ -86,16 +82,13 @@
 
   let file = dir . '/日本語のファイル.txt'
 
-  call mkdir(dir)
-  call writefile([], file)
+  call mkdir(dir, 'D')
+  call writefile([], file, 'D')
 
   let sfile = fnamemodify(file, ':8')
 
   call assert_notequal(file, sfile)
   call assert_match('\~', sfile)
-
-  call delete(file)
-  call delete(dir, 'd')
 endfunc
 
 func Test_ColonEight_notexists()