BUG: TestXMLHierarchicalBoxDataFileConverter leak

TestXMLHierarchicalBoxDataFileConverter failed to release the memory
from vtkTestUtilities::GetArgOrEnvOrDefault.

Change-Id: I24bb1e7cb20a9789db048b4cdaff81e91a4701f4
Merge topic 'Leak_TestXMLHierarchicalBoxDataFileConverter' into master

f1bf052e BUG: TestXMLHierarchicalBoxDataFileConverter leak