Clean up shared test functions, ensure coverage of edge cases

[Sparks29032]

• If any code is reused in tests, put it all in a function. If a particular function is shared across multiple test files, put it in the conftest.
  • Rethink names of functions (e.g. isfloat -> is_float, are_files_same -> files_same). This is to mirror numpy's all_close function naming.
• Think up edge cases and cover them. One important one: smear-pdf when the target and objective g(r) functions are on different grids. This has not been covered in our tests.