tests: kernel: timer_behavior: Fix building on targets with small SRAM

In the default configuration of the test, with 10000 timer samples,
the `periodic_data` array is too big to fit in SRAM on many targets.
Use lower counts of samples for those, depending on their SRAM size,
leaving at least 8 kB for other variables, buffers, stacks etc.
Exclude the test for targets with less than 16 kB.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>