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>
diff --git a/test/py/multiplexed_log.css b/test/py/multiplexed_log.css
index f135b10..9b7c44f 100644
--- a/test/py/multiplexed_log.css
+++ b/test/py/multiplexed_log.css
@@ -62,6 +62,10 @@
     color: #8080ff
 }
 
+.timestamp {
+    color: #8080ff
+}
+
 .status-pass {
     color: #00ff00
 }