run-op-bvt: use Time::localtime rather than `date` calls
run-op-bvt: use Time::localtime rather than `date` calls

we don't need to fork() and exec() just to pretty print a date.

this commit changes the format of the start/end log message from
YYYY-Month-DD to YYYY-MM-DD to be the same as other places in the log.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework