To synchronize communication via shared memory between ISR context and
thread context it is a common misconception that `volatile` is
sufficient. This is however is not the cause and the cause of many
subtle data race bugs. This fixes the issue.
By default the periph_timer tests wants to init all timers with 1MHz,
which is not suitable for all timers, e.g. the atmega timers cannot
run at that speed to make the test work they need to be set to 250kHz.
