C#: Add test for supported sinks query.

Author: michaelnebel

csharp/ql/test/query-tests/Telemetry/SupportedExternalSinks/SupportedExternalSinks.cs

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+using System;
+using System.Web;
+
+public class SupportedExternalSinks
+{
+    public void M1()
+    {
+        var o = new object();
+        var response = new HttpResponse();
+        response.AddHeader("header", "value");
+        response.AppendHeader("header", "value");
+        response.Write(o); // Known sink.
+        response.WriteFile("filename"); // Known sink.
+        response.Write(o); // Known sink.
+    }
+}

csharp/ql/test/query-tests/Telemetry/SupportedExternalSinks/SupportedExternalSinks.expected

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+| System.Web.cs#System.Web#HttpResponse.Write(object) | 2 |
+| System.Web.cs#System.Web#HttpResponse.WriteFile(string) | 1 |

csharp/ql/test/query-tests/Telemetry/SupportedExternalSinks/SupportedExternalSinks.qlref

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+Telemetry/SupportedExternalSinks.ql

csharp/ql/test/query-tests/Telemetry/SupportedExternalSinks/options

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+semmle-extractor-options: /r:System.Collections.Specialized.dll
+semmle-extractor-options: ${testdir}/../../../resources/stubs/System.Web.cs