Split up unit tests

[mrp089]

Problem

All our unit tests are in one long (>2000 lines) .cpp file. This makes it hard to see what we're currently testing and intimidating to add new tests.

Solution

Split up the test file logically into many .cpp files (like we do for integration tests). @arsLibera added code for the 1D solver to collect several unit test files.

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines

[aabrown100-git]

I agree. Also, currently our "unit tests" are rather complicated tests of the solid material models (this was my doing). These might better be split up and placed in a folder called something more appropriate, and leave "unit tests" for true unit tests.