test: adjust time-sensitive tests (#9220)

- window_split_tab_spec.lua: Put cursor at bottom of :terminal buffer so
  that it follows output.
- inccommand_spec.lua: Increase timeout to allow 2nd retry.
- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b
  ref de13113dc1
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.

src/nvim/testdir/test_lambda.vim

@@ -31,11 +31,11 @@ function! Test_lambda_with_timer()
   endfunction
 
   call s:Foo()
-  sleep 200ms
+  sleep 210ms
   " do not collect lambda
   call garbagecollect()
   let m = s:n
-  sleep 200ms
+  sleep 210ms
   call timer_stop(s:timer_id)
   call assert_true(m > 1)
   call assert_true(s:n > m + 1)