Java: Add negative model CSV validation test.

Author: michaelnebel

java/ql/test/library-tests/dataflow/external-models/negativesummaries.expected

@@ -0,0 +1,8 @@
+/**
+ * CSV Validation of negative summaries.
+ */
+
+import java
+import semmle.code.java.dataflow.ExternalFlow
+import CsvValidation
+import semmle.code.java.dataflow.internal.NegativeSummary