ZTS: include microsecond timestamps on all output

When reviewing test output after a failure, it's often quite difficult
to work out the order and timing of events, and to correlate test suite
output with kernel logs.

This adds timestamps to ZTS output to help with this, in three places:

• all of the standard log_XXX functions ultimately end up in _printline, which now prefixes output with a timestamp. An escape hatch environment variable is provided for user_cmd, which often calls the logging functions while also depending on the captured output.

• the test runner logging function log() also now prefixes its output with a timestamp.

• on failure, when capturing the kernel log in zfs_dmesg.ksh, the "iso" time format is requested.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17045