csharp: Add EFL_BETA guards around new test

csharp: Add EFL_BETA guards around new test

Summary:
The test method is not generated when beta is disabled as
`Eina.Value_Type` is marked @beta and eolian complains if we try to use
it.

Other `Eina.Value` methods work despite `Eina.Value` also being beta due
to its usage as stable through the keyword `any_value[_ptr]`.

Reviewers: vitor.sousa, bu5hm4n, felipealmeida

Reviewed By: vitor.sousa

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9449