Adding tests to at()

[Neko-Box-Coder]

Trying to add tests to ConstImpl at() const in node.hpp for #411

I have no idea if what I am doing is correct or not. And there's a TODO on line 1029 since I am not sure what to replace it with.
If this is totally wrong 😅 , I can close this PR.

I doubt I can help much with adding tests with my limited knowledge on the code base and test framework.
However, I can take a look at the whole PR and test it when it is finalized.

[biojppm]

Nice, this is good, saves me some work and helps!

The only point is that verify_assertion() should be really the (non-existent) verify_error(), which in turn instead of ExpectError::check_assertion() would call ExpectError::do_check() with the same syntax. This is because we want to verify that those calls result in an error callback regardless of RYML_USE_ASSERT; verify_assertion() checks for the former.

[Neko-Box-Coder]

Thanks for the feedback, I have added verify_error() to be used for at(). Makes sense that at() should always check (and callback) for any invalid inputs.

Do I need to add anything else? And what do I replace the capacity() with?

[biojppm]

Thanks! Don't worry about the capacity. I will dig into it.

[biojppm]

Thanks, it's good and it saved me some time!