C#: Add tests for abstract/virtual modifier of interface members

Author: tamasvajk

csharp/ql/test/library-tests/modifiers/Effectively.expected

@@ -21,3 +21,6 @@
 | Modifiers.cs:54:52:54:54 | set_P1 | public |
 | Modifiers.cs:55:20:55:21 | P2 | public |
 | Modifiers.cs:55:36:55:38 | get_P2 | public |
+| Modifiers.cs:60:22:60:23 | I1 | public |
+| Modifiers.cs:62:14:62:15 | M1 | public |
+| Modifiers.cs:63:14:63:15 | M2 | public |

csharp/ql/test/library-tests/modifiers/Modifiers.cs

@@ -56,4 +56,10 @@ public struct S
         /*private*/
         int P3 { /*private*/ get; /*private*/ set; }
     }
+
+    public interface I1
+    {
+        void M1();
+        void M2() => throw null;
+    }
 }

csharp/ql/test/library-tests/modifiers/Modifiers.expected

@@ -44,3 +44,8 @@
 | Modifiers.cs:57:13:57:14 | P3 | file://:0:0:0:0 | private |
 | Modifiers.cs:57:30:57:32 | get_P3 | file://:0:0:0:0 | private |
 | Modifiers.cs:57:47:57:49 | set_P3 | file://:0:0:0:0 | private |
+| Modifiers.cs:60:22:60:23 | I1 | file://:0:0:0:0 | public |
+| Modifiers.cs:62:14:62:15 | M1 | file://:0:0:0:0 | public |
+| Modifiers.cs:62:14:62:15 | M1 | file://:0:0:0:0 | virtual |
+| Modifiers.cs:63:14:63:15 | M2 | file://:0:0:0:0 | public |
+| Modifiers.cs:63:14:63:15 | M2 | file://:0:0:0:0 | virtual |