Add unit test for a long running timer

Rename lwip_sys_timers_get_next_timout() to sys_timeouts_get_next_timeout()

Remove Simon's compile fix in test_timers.c, it is not needed any more. The function is now used.

Once again: Fix build warnings in test_timers.c

Make _cyclic_ timers interval more deterministic - next timeout is calculated from last due time instead of relative to current time
This eliminates the cyclic timer jitter

Fix bug #52748: the bug in timeouts.c by reimplementing timer logic to use absolute instead of relative timeout values

Try to silence unused function warning in travis

Add test for bug 52748
Disabled :-) -> travis doesn't complain

Work on timer unit tests

Start implementing unit tests for timers