tests: standardise on no-arg uname with *) case for illumos

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13259

scripts/zfs-tests.sh

@@ -53,7 +53,7 @@ TAGS=""
 ITERATIONS=1
 ZFS_DBGMSG="$STF_SUITE/callbacks/zfs_dbgmsg.ksh"
 ZFS_DMESG="$STF_SUITE/callbacks/zfs_dmesg.ksh"
-UNAME=$(uname -s)
+UNAME=$(uname)
 RERUN=""
 KMEMLEAK=""
 

scripts/zfs.sh

@@ -260,7 +260,7 @@ if [ "$(id -u)" != 0 ]; then
 	exit 1
 fi
 
-UNAME=$(uname -s)
+UNAME=$(uname)
 
 if [ "$UNLOAD" = "yes" ]; then
 	kill_zed