patch 8.0.1190: unusable after opening new window in BufWritePre event

Problem:    Vim becomes unusable after opening new window in BufWritePre
            event.
Solution:   Call not_exiting(). (Martin Tournoij, closes #2205)
            Also for "2q" when a help window is open.  Add a test.

src/testdir/test_writefile.vim

@@ -49,3 +49,47 @@ func Test_writefile_fails_conversion()
   bwipe!
   set backup& writebackup&
 endfunc
+
+func SetFlag(timer)
+  let g:flag = 1
+endfunc
+
+func Test_write_quit_split()
+  " Prevent exiting by splitting window on file write.
+  augroup testgroup
+    autocmd BufWritePre * split
+  augroup END
+  e! Xfile
+  call setline(1, 'nothing')
+  wq
+
+  if has('timers')
+    " timer will not run if "exiting" is still set
+    let g:flag = 0
+    call timer_start(1, 'SetFlag')
+    sleep 50m
+    call assert_equal(1, g:flag)
+    unlet g:flag
+  endif
+  au! testgroup
+  bwipe Xfile
+  call delete('Xfile')
+endfunc
+
+func Test_nowrite_quit_split()
+  " Prevent exiting by opening a help window.
+  e! Xfile
+  help
+  wincmd w
+  exe winnr() . 'q'
+
+  if has('timers')
+    " timer will not run if "exiting" is still set
+    let g:flag = 0
+    call timer_start(1, 'SetFlag')
+    sleep 50m
+    call assert_equal(1, g:flag)
+    unlet g:flag
+  endif
+  bwipe Xfile
+endfunc