Added test for failing nested given validation

[andreacfm]

Using nested given validations looks unsupported. This is just a simple test case to show the issue.

[grape-bot]

	1 Warning
:warning:	Unless you’re refactoring existing code, please update CHANGELOG.md.

	1 Message
:book:	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#1804](https://github.com/ruby-grape/grape/pull/1804): Added test for failing nested given validation - [@andreacfm](https://github.com/andreacfm).

Generated by :no_entry_sign: danger

[dblock]

@darren987469 was just in this code in #1810, maybe interested in taking a look at this too?

[andreacfm]

@dblock I will have a look. Thanks

[numbata]

Look like can be closed :)

[dblock]

Yep, closing with https://github.com/ruby-grape/grape/pull/1811.