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

tests/zfs-tests/include/libtest.shlib

@@ -3332,7 +3332,7 @@ function zed_stop
 
 	log_note "Stopping ZED"
 	if [[ -f ${ZEDLET_DIR}/zed.pid ]]; then
-		zedpid=$(cat ${ZEDLET_DIR}/zed.pid)
+		zedpid=$(<${ZEDLET_DIR}/zed.pid)
 		kill $zedpid
 		while ps -p $zedpid > /dev/null; do
 			sleep 1