patch 9.0.0768: 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_visual.vim b/src/testdir/test_visual.vim
index a7f8ec4..c6f80aa 100644
--- a/src/testdir/test_visual.vim
+++ b/src/testdir/test_visual.vim
@@ -1274,7 +1274,7 @@
     set virtualedit=block
     normal G
   END
-  call writefile(lines, 'XTest_block')
+  call writefile(lines, 'XTest_block', 'D')
 
   let buf = RunVimInTerminal('-S XTest_block', {'rows': 8, 'cols': 50})
   call term_sendkeys(buf, "\<C-V>gg$")
@@ -1286,7 +1286,6 @@
   " clean up
   call term_sendkeys(buf, "\<Esc>")
   call StopVimInTerminal(buf)
-  call delete('XTest_block')
 endfunc
 
 func Test_visual_block_ctrl_w_f()
@@ -1331,11 +1330,10 @@
 
       :
   END
-  call writefile(lines, 'XvisualReselect')
+  call writefile(lines, 'XvisualReselect', 'D')
   source XvisualReselect
 
   bwipe!
-  call delete('XvisualReselect')
 endfunc
 
 func Test_visual_block_insert_round_off()