[jacobmerson]

This PR is based on #109 and will look much more sensible after that is merged. Please use merge instead of rebase for that PR as otherwise it'll make merging this PR more challenging.

The goal of this PR is to reduce the repeated test code and make the tests more consistent in terms of what's tested across the different backends/element types.

Once #109 is merged, this PR should net roughly -1800 lines of code despite expanding the coverage and making it easier to add tests for new controllers, element types, etc.

[jacobmerson]

When this gets reviewed, please consider adding Catch2 as a dependency and using a proper test framework rather than this one that Claude and I came up with. Here is the example of what some of those tests look like with Catch2 instead. But Catch2 also has matchers for comparing floating point values, better reporting of expected values, integration with cmake, etc.

// testController.cpp
#ifdef MESHFIELDS_ENABLE_CABANA
#define CONTROLLERS (KokkosRanks, CabanaRanks)
#else
#define CONTROLLERS (KokkosRanks)
#endif

TEMPLATE_PRODUCT_TEST_CASE("fields of each rank", "[controller]",
                           CONTROLLERS, (int, double)) {
  auto c = createRanks<TestType>();          // name: "... - KokkosRanks<int>"
  SECTION("rank 1") { testRank<1>(c, rank1); }
  SECTION("rank 2") { testRank<2>(c, rank2); }
  // ...
}

template <int R, typename Ctrlr> void testRank(Ctrlr &c, Extents n) {
  auto field = MeshField::makeField<Ctrlr, R - 1>(c);
  CHECK(extentMismatches<R>(field, n) == 0);
  fill<R>(field, n);
  CHECK(mismatches<R>(field, n) == 0);       // failure prints "3 == 0"
}

For the case with several axes (reproduction):

template <template <typename...> class C, size_t D, class E> struct Cfg {
  template <typename... Ts> using Controller = C<Ts...>;
  static constexpr size_t dim = D; using Element = E;
};
using Configs = CartesianProduct<Controllers, Dims, Elements>; // std::tuple<Cfg<...>...>

TEMPLATE_LIST_TEST_CASE("reproduction", "[reproduction]", Configs) {
  using C = TestType;
  auto &mesh = cachedMesh<C::dim>();
  SECTION("identity") { testOrders<C>(mesh, MeshField::IdentityCoordinateMap{}); }
  SECTION("shear")    { testOrders<C>(mesh, Shear<C::dim>{}); }
}