patch 8.0.1440: terminal window: some vterm responses are delayed

Problem:    Terminal window: some vterm responses are delayed.
Solution:   After writing input. check if there is output to read. (Ozaki
            Kiichi, closes #2594)
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 9b96b49..130562f 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -619,6 +619,8 @@
   let buf = term_start([GetVimProg(), '--clean', '-c', 'set noswapfile', 'Xsearch.txt'], {'term_rows': 3})
 
   call WaitFor({-> lines == [term_getline(buf, 1), term_getline(buf, 2)] })
+  " wait for vim to complete initialization
+  call term_wait(buf)
 
   " Get attr of normal(a0), incsearch(a1), hlsearch(a2) highlight
   call term_sendkeys(buf, ":set incsearch hlsearch\<cr>")