test/py: add timestamps to log

It can be useful to record how long tests take; this can help debug slow
running test systems or track changes in performance over time. Enhance
the test system to record timestamps while running test:
- Whenever a new log file section is started.
- After U-Boot is started and communication has been established.
- After each host or U-Boot command is executed.

Signed-off-by: Stephen Warren <swarren@nvidia.com>