Build C++ unit tests against a static library

[paulromano]

Description

Another PR here working toward Windows support. Notably, in #2919 there are a bunch of OPENMC_API macros that are added on globals that I'm hoping to avoid. One category is global variables that get used in C++ tests; we can avoid those by compiling the OpenMC C++ sources once in an object library and reuse those objects for both the shared library and a static library dedicated to C++ unit tests. This keeps internal globals used by the tests from defining the shared library interface without compiling the source tree twice.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[shimwell]

Looks good to me, I had to do the same changes when prototyping an openmc windows wheel so I'm confident these changes are needed of windows support

One optional name change for your consideration.