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/tests/perf/perf.shlib

@@ -416,7 +416,7 @@ function get_system_config
 			then
 				printf ",\n" >>$config
 			fi
-			printf  "    \"$tunable\": \"$(cat $zfs_tunables/$tunable)\"" \
+			printf  "    \"$tunable\": \"$(<$zfs_tunables/$tunable)\"" \
 			    >>$config
 		done
 		printf "\n  }\n" >>$config