Ensuring only exact "vacuum" assignment is detected as vacuum

[bam241]

Description

This is a follow up on svalinn/DAGMC#805

This ensures that a assignment named "not_a_vacuum" is not flagged as vacuum and therefore is not assigned with vacuum boundary condition.

Fixes # (issue)
This is related to #3218

I also had to implement that a different way than initially expected.
I first thought about leveraging the #3056 PR recently merged to do this kind of test.
But, vacuum detection is perform (in the DAGMC workflow within openMC) before applying materials assignment (therefore before overriding)

This is why I implemented this by making temporary copy of the dagmc.hm5 and modifying them. (Not that seing how simple this is we could potentially provide a method that allows to do that within dagmc.py ?)

I trying and failed to design this test as a fixture, I don't know why it failed it never recognised the returned model as a openmc.Model but as a function... (@pshriwise if you have any insight on this ?)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

This works is sponsored by Proxima Fusion

[pshriwise]

@bam241 I took a crack at simplifying this test somewhat. I'm pretty sure it still tests what you're intending, but let me know if I'm missing something.

I don't have a suggestion on how to re-use the model fixture above here. I don't think it's too important though as the materials in this test could be anything as long as their names are set appropriately.