C#: Add test for global using directive.

Author: michaelnebel

csharp/ql/test/library-tests/csharp10/global/GlobalUsing.cs

@@ -0,0 +1,3 @@
+global using System;
+global using System.IO;
+global using static System.Text.Encoding;

csharp/ql/test/library-tests/csharp10/global/MyClass1.cs

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+using System.Collections;
+
+public class MyClass1
+{
+    public void HelloWorld()
+    {
+        Console.WriteLine("Hello World");
+    }
+}

csharp/ql/test/library-tests/csharp10/global/globalUsing.expected

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+globalUsing
+| GlobalUsing.cs:1:1:1:20 | using ...; |
+| GlobalUsing.cs:2:1:2:23 | using ...; |
+| GlobalUsing.cs:3:1:3:41 | using static ...; |
+localUsing
+| MyClass1.cs:1:1:1:25 | using ...; |
+globalUsingNamespace
+| GlobalUsing.cs:1:1:1:20 | using ...; | file://:0:0:0:0 | System |
+| GlobalUsing.cs:2:1:2:23 | using ...; | file://:0:0:0:0 | System.IO |

csharp/ql/test/library-tests/csharp10/global/globalUsing.ql

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+import csharp
+
+query predicate globalUsing(UsingDirective und) { und.isGlobal() }
+
+query predicate localUsing(UsingDirective ud) { not ud.isGlobal() }
+
+query predicate globalUsingNamespace(UsingNamespaceDirective und, Namespace namespace) {
+  und.isGlobal() and
+  namespace = und.getImportedNamespace()
+}