Add test for var local-func return type

Support for using `var` in a local function return type was already
removed in PR dotnet#7916. This PR adds a test to verify that.

Closes dotnet#10392.

src/Compilers/CSharp/Test/Semantic/Semantics/LocalFunctionTests.cs

@@ -107,6 +107,43 @@ Action local(dynamic z)
             VerifyOutput(src, "012");
         }
 
+        [Fact]
+        public void VarLocalFunction()
+        {
+            var src = @"
+class C
+{
+    void M()
+    {
+        var local() => 0;
+        int x = local();
+   } 
+}";
+            VerifyDiagnostics(src,
+                // (6,9): error CS0825: The contextual keyword 'var' may only appear within a local variable declaration or in script code
+                //         var local() => 0;
+                Diagnostic(ErrorCode.ERR_TypeVarNotFound, "var").WithLocation(6, 9));
+        }
+
+        [Fact]
+        public void VarLocalFunction2()
+        {
+            var comp = CreateCompilationWithMscorlib(@"
+class C
+{
+    private class var
+    {
+    }
+
+    void M()
+    {
+        var local() => new var();
+        var x = local();
+   } 
+}", parseOptions: DefaultParseOptions);
+            comp.VerifyDiagnostics();
+        }
+
         [Fact]
         public void EndToEnd()
         {