Skip ip6 tests when ip6 is not available

--- a/t/t2_tcp6.sh	2021-08-09 14:15:27.000000000 -0300
+++ b/t/t2_tcp6.sh	2021-10-02 23:13:59.832306995 -0300
@@ -1,6 +1,9 @@
 #!/bin/bash -e
 . $(dirname $0)/base.sh
 
+# SKIP if ip6 not available
+(< /dev/udp/$ip6/$port) 2> /dev/null || exit 77
+
 # usage:
 # run_iperf -s server args   -c client args
 #
--- a/t/t4_udp6.sh	2021-08-09 14:15:27.000000000 -0300
+++ b/t/t4_udp6.sh	2021-10-02 23:13:57.274282598 -0300
@@ -1,6 +1,9 @@
 #!/bin/bash -e
 . $(dirname $0)/base.sh
 
+# SKIP if ip6 not available
+(< /dev/udp/$ip6/$port) 2> /dev/null || exit 77
+
 # usage:
 # run_iperf -s server args   -c client args
 #