C#: Add some synthetic library extensions methods and tests for cs/dereferenced-value-is-always-null.

Author: michaelnebel

csharp/ql/test/query-tests/Nullness/F.cs

@@ -0,0 +1,16 @@
+using Library;
+
+public class F
+{
+    public void M1()
+    {
+        object o = null;
+        o.Accept(); // $ Alert[cs/dereferenced-value-is-always-null]
+    }
+
+    public void M2()
+    {
+        object? o = null;
+        o.AcceptNullable();
+    }
+}

csharp/ql/test/query-tests/Nullness/NullAlways.expected

@@ -1,3 +1,4 @@
+#select
 | A.cs:8:15:8:32 | access to local variable synchronizedAlways | Variable $@ is always null at this dereference. | A.cs:7:16:7:33 | synchronizedAlways | synchronizedAlways |
 | A.cs:17:9:17:17 | access to local variable arrayNull | Variable $@ is always null at this dereference. | A.cs:16:15:16:23 | arrayNull | arrayNull |
 | A.cs:31:27:31:37 | access to local variable arrayAccess | Variable $@ is always null at this dereference. | A.cs:26:15:26:25 | arrayAccess | arrayAccess |
@@ -38,6 +39,10 @@
 | E.cs:331:9:331:9 | access to local variable x | Variable $@ is always null at this dereference. | E.cs:330:13:330:13 | x | x |
 | E.cs:405:16:405:16 | access to local variable i | Variable $@ is always null at this dereference. | E.cs:403:14:403:14 | i | i |
 | E.cs:439:13:439:13 | access to parameter s | Variable $@ is always null at this dereference. | E.cs:435:29:435:29 | s | s |
+| F.cs:8:9:8:9 | access to local variable o | Variable $@ is always null at this dereference. | F.cs:7:16:7:16 | o | o |
+| F.cs:14:9:14:9 | access to local variable o | Variable $@ is always null at this dereference. | F.cs:13:17:13:17 | o | o |
 | Forwarding.cs:36:31:36:31 | access to local variable s | Variable $@ is always null at this dereference. | Forwarding.cs:7:16:7:16 | s | s |
 | Forwarding.cs:40:27:40:27 | access to local variable s | Variable $@ is always null at this dereference. | Forwarding.cs:7:16:7:16 | s | s |
 | NullAlwaysBad.cs:9:30:9:30 | access to parameter s | Variable $@ is always null at this dereference. | NullAlwaysBad.cs:7:29:7:29 | s | s |
+testFailures
+| F.cs:14:9:14:9 | Variable $@ is always null at this dereference. | Unexpected result: Alert |

csharp/ql/test/query-tests/Nullness/options

@@ -1,3 +1,4 @@
 semmle-extractor-options: /nostdlib /noconfig
 semmle-extractor-options: --load-sources-from-project:${testdir}/../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj
-semmle-extractor-options: ${testdir}/../../resources/stubs/Microsoft.VisualStudio.TestTools.UnitTesting.cs
+semmle-extractor-options: ${testdir}/../../resources/stubs/Microsoft.VisualStudio.TestTools.UnitTesting.cs
+semmle-extractor-options: ${testdir}/../../resources/stubs/Library.cs

csharp/ql/test/resources/stubs/Library.cs

@@ -0,0 +1,13 @@
+namespace Library;
+
+/*
+ * This file is for making stubs for library methods used for testing purposes.
+ * The file is located in the stubs folder, because then the code is not
+ * recognized as being from source.
+ */
+public static class MyExtensions
+{
+    public static void Accept(this object o) { }
+
+    public static void AcceptNullable(this object? o) { }
+}