Enabled a test previously failing on Linux because of old Mono.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

tests/CSharp/CSharp.Tests.cs

@@ -388,8 +388,7 @@ public unsafe class CSharpTests : GeneratorTestFixture
         Assert.AreEqual(dervClass.M, 2);
     }
     
-    // TODO: fails on the Linux CI but works locally on Vagrant - both have the same Mono version; it also works on OS X; go figure
+    [Test]
-    [Test, Platform(Exclude = "Linux")]
     public unsafe void TestMultiOverLoadPtrToRef()
     {
         var r = 0;