<mdspan>: Add separate tests for extents and default_accessor

[JMazurkiewicz]

Short motivation:

• Separate tests are easier to read and maintain,
• We avoid files with 1000 lines of code.

That said, this PR adds:

• Add separate tests for extents and default_accessor (I'll introduce the rest of the tests in other PR),
• Drive-by: restore <type_traits> include in <mdspan> (see <mdspan>: Use concepts everywhere and add mdspan deduction guides #3564 (comment)).

P0009R18_mdspan_extents/test.cpp tests have two FIXME comment, as there might be a bug in extents implementation

[StephanTLavavej]

Thanks for extending this test coverage! 😹