fix tests

Author: ty-v1

src/test/java/jp/kusumotolab/kgenprog/output/BaseSerializerTest.java

@@ -140,6 +140,7 @@ private void assertBase(final Base base, final String serializedBase) {
             JsonKeyAlias.Base.SNIPPET);
 
     assertThatJson(serializedBase).node(JsonKeyAlias.Base.FILE_NAME)
+        .isString()
         .isEqualTo(expectFqn);
 
     assertThatJson(serializedBase).node(JsonKeyAlias.Base.LINE_NUMBER_RANGE)