digdelv: Extra quotes prevent IPv6 runs

Portion of the digdelv test are skipped on IPv6 due to extra quotes
around $TESTSOCK6: "I:digdelv:IPv6 unavailable; skipping".

Researched by @michal.

Regressed with 351efd8812.

bin/tests/system/staticstub/tests.sh

@@ -160,7 +160,7 @@ n=`expr $n + 1`
 # Note: for a short term workaround we use ::1, assuming it's configured and
 # usable for our tests.  We should eventually use the test ULA and available
 # checks introduced in change 2916.
-if $TESTSOCK6 ../testsock6.pl ::1 2> /dev/null
+if testsock6 ::1
 then
     echo_i "checking IPv6 static-stub address ($n)"
     ret=0