ZTS: Fix mmp_interval failure

The mmp_interval test case was failing on Fedora 30 due to the built-in
'echo' command terminating the script when it was unable to write to
the sysfs module parameter.  This change in behavior was observed with
ksh-2020.0.0-alpha1.  Resolve the issue by using the external cat
command which fails gracefully as expected.

Additionally, remove some incorrect quotes around the $? return values.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8906

tests/zfs-tests/include/libtest.shlib

@@ -3494,13 +3494,13 @@ function set_tunable_impl
 	Linux)
 		typeset zfs_tunables="/sys/module/$module/parameters"
 		[[ -w "$zfs_tunables/$tunable" ]] || return 1
-		echo -n "$value" > "$zfs_tunables/$tunable"
-		return "$?"
+		cat >"$zfs_tunables/$tunable" <<<"$value"
+		return $?
 		;;
 	SunOS)
 		[[ "$module" -eq "zfs" ]] || return 1
 		echo "${tunable}/${mdb_cmd}0t${value}" | mdb -kw
-		return "$?"
+		return $?
 		;;
 	esac
 }
@@ -3527,7 +3527,7 @@ function get_tunable_impl
 		typeset zfs_tunables="/sys/module/$module/parameters"
 		[[ -f "$zfs_tunables/$tunable" ]] || return 1
 		cat $zfs_tunables/$tunable
-		return "$?"
+		return $?
 		;;
 	SunOS)
 		[[ "$module" -eq "zfs" ]] || return 1