<mdspan>: Remove old tests

[JMazurkiewicz]

The main goal of this PR is to remove old tests implemented by Matt Stephanson:

• Remove tests/std/tests/P0009R18_mdspan directory,
• Preserve some tests from P0009R18_mdspan that were not present in new fragmented test suite,
• Fix layout_right's constructor noexcept specifier,
• test_mdspan_support.hpp: implement get_mapping_properties function that manually computes properties of layout mapping. This is based on Matt Stephanson's TestMapping function:

  STL/tests/std/tests/P0009R18_mdspan/test.cpp

  Lines 344 to 430 in ab7bb23

  	template <class Mapping, enable_if_t<Mapping::extents_type::rank() == 2, int> = 0>
  	void TestMapping(const Mapping& map) {
  	using IndexT = typename Mapping::index_type;
  	using RankT = typename Mapping::rank_type;
  	static_assert(is_same_v<IndexT, decltype(declval<Mapping>()(IndexT{0}, IndexT{0}))>);
  	static_assert(is_same_v<IndexT, decltype(declval<Mapping>().stride(RankT{0}))>);
  	array<IndexT, Mapping::extents_type::rank()> s;
  	const auto& e = map.extents();
  	size_t num_entries = 1;
  	for (size_t i = 0; i < Mapping::extents_type::rank(); ++i) {
  	num_entries *= e.extent(i);
  	s[i] = map.stride(i);
  	}
  	vector<IndexT> indices;
  	indices.reserve(num_entries);
  	for (IndexT i = 0; i < e.extent(0); ++i) {
  	for (IndexT j = 0; j < e.extent(1); ++j) {
  	const auto idx = i * s[0] + j * s[1];
  	assert(map(i, j) == idx);
  	indices.push_back(idx);
  	}
  	}
  	bool is_unique = true;
  	bool is_exhaust = true;
  	sort(indices.begin(), indices.end());
  	for (size_t i = 1; i < indices.size(); ++i) {
  	const auto diff = indices[i] - indices[i - 1];
  	if (diff == 0) {
  	is_unique = false;
  	} else if (diff != 1) {
  	is_exhaust = false;
  	}
  	}
  	assert(map.is_unique() == is_unique);
  	assert(map.is_exhaustive() == is_exhaust);
  	assert(map.required_span_size() == indices.back() + 1);
  	}
  	template <class Mapping, enable_if_t<Mapping::extents_type::rank() == 3, int> = 0>
  	void TestMapping(const Mapping& map) {
  	using IndexT = typename Mapping::index_type;
  	using RankT = typename Mapping::rank_type;
  	static_assert(is_same_v<IndexT, decltype(declval<Mapping>()(IndexT{0}, IndexT{0}, IndexT{0}))>);
  	static_assert(is_same_v<IndexT, decltype(declval<Mapping>().stride(RankT{0}))>);
  	array<IndexT, Mapping::extents_type::rank()> s;
  	const auto& e = map.extents();
  	size_t num_entries = 1;
  	for (size_t i = 0; i < Mapping::extents_type::rank(); ++i) {
  	num_entries *= e.extent(i);
  	s[i] = map.stride(i);
  	}
  	vector<IndexT> indices;
  	indices.reserve(num_entries);
  	for (IndexT i = 0; i < e.extent(0); ++i) {
  	for (IndexT j = 0; j < e.extent(1); ++j) {
  	for (IndexT k = 0; k < e.extent(2); ++k) {
  	const auto idx = i * s[0] + j * s[1] + k * s[2];
  	assert(map(i, j, k) == idx);
  	indices.push_back(idx);
  	}
  	}
  	}
  	bool is_unique = true;
  	bool is_exhaust = true;
  	sort(indices.begin(), indices.end());
  	for (size_t i = 1; i < indices.size(); ++i) {
  	const auto diff = indices[i] - indices[i - 1];
  	if (diff == 0) {
  	is_unique = false;
  	} else if (diff != 1) {
  	is_exhaust = false;
  	}
  	}
  	assert(map.is_unique() == is_unique);
  	assert(map.is_exhaustive() == is_exhaust);
  	assert(map.required_span_size() == indices.back() + 1);
  	}

• Drive-by: include <test_mdspan_support.hpp> instead of "test_mdspan_support.hpp" in tests,
• Overall test coverage improvements.

[StephanTLavavej]

FYI, I edited your PR description to permalink the TestMapping citation, because a non-permalink will be invalidated after this PR removes that directory.

[StephanTLavavej]

Thanks! I pushed a conflict-free merge to pick up the recent toolset update, followed by simple changes to fix static_asserts that I'm 99.9% sure about - please meow if I was confused. I'll merge this after the tests pass.