tests: Split ipv6 specific tests into a new test group

This way they can be easily disabled later on when IPv6 support is made
optional.

Change-Id: I3906dbf55ccf1650083398e08ac870add0bbdcef
diff --git a/tests/lib/ippool_v6_test.err b/tests/lib/ippool_v6_test.err
new file mode 100644
index 0000000..f0edefe
--- /dev/null
+++ b/tests/lib/ippool_v6_test.err
@@ -0,0 +1,2 @@
+No more IP addresses available
+No more IP addresses available