C#: Add a consistency test for getAPrimaryQlClass

csharp/ql/consistency-queries/PrimaryQlClass.ql

@@ -0,0 +1,6 @@
+import csharp
+
+query predicate missingPrimaryQlClass(Element e) {
+  e.getAPrimaryQlClass() = "???" and
+  e.fromSource()
+}