Ignore NcML file meant for modificaiton testing

modifyNestedGroups.xml is intended to apply to an already existing NetcdfDataset object using NcmlReader.wrapNcml, and isn't appropriate for use by the TestNcmlReadersComnpare test, which expects the NcML to fully define a dataset or reference one directly in the document.

cdm/core/src/test/java/ucar/nc2/ncml/TestNcmlReadersCompare.java

@@ -78,6 +78,9 @@ public boolean accept(File pathname) {
       // NcMLReader does not change variable to type int, so fails.
       if (name.contains("aggSynthetic.xml"))
         return false;
+      // Simulates a TDS datasetScan using NcML and does not fully define a dataset.
+      if (name.contains("modifyNestedGroups.xml"))
+        return false;
       // Bug in old reader
       if (name.contains("testStandaloneNoEnhance.ncml"))
         return false;