patch 8.1.1691: diff test fails on some systems

Problem:    Diff test fails on some systems. (Elimar Riesebieter)
Solution:   Add a term_wait() call.

src/testdir/test_diffmode.vim

@@ -743,6 +743,7 @@ func VerifyInternal(buf, dumpfile, extra)
   call term_sendkeys(a:buf, ":diffupdate!\<CR>")
   " trailing : for leaving the cursor on the command line
   call term_sendkeys(a:buf, ":set diffopt=internal,filler" . a:extra . "\<CR>:")
+  call term_wait(a:buf)
   call VerifyScreenDump(a:buf, a:dumpfile, {})
 endfunc