[Bug] provided explaination for static closures

[peterfox]

Currently, if you try to set up a test with a static closure the test fails but the description is somewhat vague and points to a Laravel class for some reason.

What:

• [x] Bug Fix
• [ ] New Feature

Description:

This PR aims to give a dedicated exception for when a closure is provided that is static.

Related:

~~I appreciate this PR adds an extra dependency to be able to perform reflections on the closure itself. I would personally say it's worthwhile as it's a Laravel package and it doesn't carry any additional dependencies itself.~~

Realised I could use ReflectFunction instead