tests: kernel: timer_behavior: improve timer_tick_train output

Print the "perfect" reference period for easier evaluation.
Suggest a remedy to the missed ticks problem.

Still, that wasn't satisfactory. Implemented a count of missed ticks
to get to the bottom of this issue. Found that missed ticks always came
to a perfect count of 40.

Incidentally, the busy loop prints a line every 250 ms and the test spans
10 seconds. There are no such coincidences.

Turns out that CONFIG_PRINTK_SYNC was set by default. This disables IRQs
for the serial output duration, which can be quite long at 115200 bauds.
Given a 60-ish character line length, this represents more than 5 ms of
no IRQ servicing during a timer latency measurement test which is bad.
So make sure CONFIG_PRINTK_SYNC=n for proper statistics.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>