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/callbacks/zfs_mmp.ksh

@@ -18,7 +18,7 @@
 
 # $1: number of lines to output (default: 40)
 typeset lines=${1:-40}
-typeset history=$(cat /sys/module/zfs/parameters/zfs_multihost_history)
+typeset history=$(</sys/module/zfs/parameters/zfs_multihost_history)
 
 if [ $history -eq 0 ]; then
 	exit