re-enable ignored netcore tests

Author: Eirik Tsarpalis

tests/FsPickler.Tests/SerializerTests.fs

@@ -376,9 +376,6 @@ type SerializationTests (fixture : ISerializerFixture) =
             let e = try edi.Throw() ; failwith "impossible" with e -> e
             e.StackTrace.Split('\n').Length |> should be (greaterThan 20)
 
-#if NETCOREAPP2_0
-    [<Ignore("Test is failing in CI due to older versions of netcore.")>]
-#endif
     [<Test; Category("Generic BCL Types")>]
     member __.``BCL: misc exceptions`` () =
         __.TestException <| new InvalidOperationException()