test: adjust time-sensitive tests

From test_alot.vim:
    Found errors in Test_lambda_with_timer():
    First run:
    function RunTheTest[35]..Test_lambda_with_timer line 19: Expected True but got 0
    Second run:
    function RunTheTest[35]..Test_lambda_with_timer line 19: Expected True but got 0

previously: #9220

- 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.
- Call test_garbagecollect_now(), as Vim does.

src/nvim/testdir/test_lambda.vim

@@ -33,9 +33,9 @@ function! Test_lambda_with_timer()
   call s:Foo()
   sleep 210ms
   " do not collect lambda
-  call garbagecollect()
+  call test_garbagecollect_now()
   let m = s:n
-  sleep 210ms
+  sleep 230ms
   call timer_stop(s:timer_id)
   call assert_true(m > 1)
   call assert_true(s:n > m + 1)