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.

test/functional/helpers.lua

@@ -300,8 +300,10 @@ end
 -- Calls fn() until it succeeds, up to `max` times or until `max_ms`
 -- milliseconds have passed.
 local function retry(max, max_ms, fn)
+  assert(max == nil or max > 0)
+  assert(max_ms == nil or max_ms > 0)
   local tries = 1
-  local timeout = (max_ms and max_ms > 0) and max_ms or 10000
+  local timeout = (max_ms and max_ms or 10000)
   local start_time = luv.now()
   while true do
     local status, result = pcall(fn)