C#: Add some QL tests for extension types.

Author: michaelnebel

csharp/ql/test/library-tests/extension/extensionType.expected

@@ -0,0 +1,5 @@
+extensionTypeReceiverParameter
+| extensions.cs:6:5:16:5 | extension(String) | extensions.cs:6:22:6:22 | s |
+extensionTypeExtendedType
+| extensions.cs:6:5:16:5 | extension(String) | string |
+| extensions.cs:18:5:23:5 | extension(Object) | object |

csharp/ql/test/library-tests/extension/extensionType.ql

@@ -0,0 +1,11 @@
+import csharp
+
+query predicate extensionTypeReceiverParameter(ExtensionType et, Parameter p) {
+  et.getFile().getBaseName() = "extensions.cs" and
+  p = et.getReceiverParameter()
+}
+
+query predicate extensionTypeExtendedType(ExtensionType et, string t) {
+  et.getFile().getBaseName() = "extensions.cs" and
+  t = et.getExtendedType().toStringWithTypes()
+}