C#: Add test for known sources telemetry query.

Author: michaelnebel

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

@@ -0,0 +1,12 @@
+using System;
+
+public class SupportExternalSources
+{
+    public void M1()
+    {
+        var l1 = Console.ReadLine(); // Known source.
+        var l2 = Console.ReadLine(); // Known source.
+        Console.SetError(Console.Out);
+        var x = Console.Read(); // Know source.
+    }
+}

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

@@ -0,0 +1,2 @@
+| System.Console.dll#System#Console.ReadLine() | 2 |
+| System.Console.dll#System#Console.Read() | 1 |

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

@@ -0,0 +1 @@
+Telemetry/SupportedExternalSources.ql