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_source.vim b/src/testdir/test_source.vim
index 51de0e3..3692eee 100644
--- a/src/testdir/test_source.vim
+++ b/src/testdir/test_source.vim
@@ -51,12 +51,10 @@
 " When deleting a file and immediately creating a new one the inode may be
 " recycled.  Vim should not recognize it as the same script.
 func Test_different_script()
-  call writefile(['let s:var = "asdf"'], 'XoneScript')
+  call writefile(['let s:var = "asdf"'], 'XoneScript', 'D')
   source XoneScript
-  call delete('XoneScript')
-  call writefile(['let g:var = s:var'], 'XtwoScript')
+  call writefile(['let g:var = s:var'], 'XtwoScript', 'D')
   call assert_fails('source XtwoScript', 'E121:')
-  call delete('XtwoScript')
 endfunc
 
 " When sourcing a vim script, shebang should be ignored.