patch 9.0.0626: 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_mksession.vim b/src/testdir/test_mksession.vim
index 92cf8bf..1ce5a28 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -1098,14 +1098,13 @@
   bwipe!
 
   " Recreate the swap file to pretend the file is being edited
-  call writefile(cont, fname)
+  call writefile(cont, fname, 'D')
   set shortmess+=A
   source Xtestsession
 
   set shortmess&
   set sessionoptions&
   call delete('Xtestsession')
-  call delete(fname)
 endfunc
 
 " Test for mksession with 'compatible' option
@@ -1235,7 +1234,7 @@
 
 " Test for creating views with manual folds
 func Test_mkview_manual_fold()
-  call writefile(range(1,10), 'Xmkvfile')
+  call writefile(range(1,10), 'Xmkvfile', 'D')
   new Xmkvfile
   " create recursive folds
   5,6fold
@@ -1259,7 +1258,6 @@
   source Xview
   call assert_equal([-1, -1, -1, -1, -1, -1], [foldclosed(3), foldclosed(4),
         \ foldclosed(5), foldclosed(6), foldclosed(7), foldclosed(8)])
-  call delete('Xmkvfile')
   call delete('Xview')
   bw!
 endfunc