If address/get_if_addrs6 can't findipv6, log WARN, not ERR

Fixes 29160, and allows 28668 (treating ERR logs as test failures)
to procede.

changes/ticket29160

@@ -0,0 +1,4 @@
+  o Minor bugfixes (tests):
+    - Do not log an error-level message if we fail to find an IPv6
+      network interface from the unit tests. Fixes bug 29160; bugfix on
+      0.2.7.3-rc.

src/test/test_address.c

@@ -1013,7 +1013,7 @@ test_address_get_if_addrs6(void *arg)
 
   (void)arg;
 
-  rv = get_interface_address6(LOG_ERR, AF_INET6, &tor_addr);
+  rv = get_interface_address6(LOG_WARN, AF_INET6, &tor_addr);
 
   /* Work even on systems without IPv6 interfaces */
   if (rv == 0) {
@@ -1204,4 +1204,3 @@ struct testcase_t address_tests[] = {
   ADDRESS_TEST(tor_addr_in_same_network_family, 0),
   END_OF_TESTCASES
 };
-