[2906] a bit more comment about test setup

src/lib/datasrc/tests/zone_table_accessor_unittest.cc

@@ -40,6 +40,8 @@ namespace {
 class ZoneTableAccessorTest : public ::testing::Test {
 protected:
     ZoneTableAccessorTest() :
+        // The paths of the zone files are dummy and wouldn't even exist,
+        // but it doesn't matter in this test.
         config_spec_(Element::fromJSON(
                          "{\"cache-enable\": true,"
                          " \"params\": "