Add trivial termios tests.

...and fix the bugs.

Also explain why we can't support separate input and output speeds
without an ABI change. Luckily no-one is likely to need that anyway,
and they can always work around it by using `struct termios2` directly
themselves.

Bug: http://b/69816452
Test: ran tests
Change-Id: Ie08499a198bb6a20d7e5e2f5ff74a60bd53e97e1