ZTS: change $(cat) to $(<) for speedup

It's better to use ksh/bash built in methods,
rather than spawn new processes every time.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Wren Kennedy <john.kennedy@delphix.com>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #8071