ZTS: Simplify some libtest functions

Don't echo the results of arithmetic expressions, it's not necessary.

Use hw.clockrate sysctl to get CPU freq instead of parsing dmesg.boot
for a line that might not even be there anymore.

Reduce bookkeeping in fill_fs, making it easier to follow.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10113