tests: kernel: timer_behavior_external: Additional stats recording

Additional logging of kernel.timer.timer_behavior_external test case
statistics for timer drift, variance, etc. as JSON-formatted records
to make easier data collection and its further analysis.
These log records will be processed by the Twister Harness recording
feature which captures and parses timer statistics from the log output,
then composes it into twister.json and recording.csv files.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>