libstdc++: Add tests for std::extents.

libstdc++: Add tests for std::extents.

A prior commit added std::extents, this commit adds the tests. The bulk
is focussed on testing the constructors. These are split into three
groups:

1. the ctor from other extents and the copy ctor,
2. the ctor from a pack of integer-like objects,
3. the ctor from shapes, i.e. span and array.

For each group check that the ctor:
* produces an object with the expected values for extent,
* is implicit if and only if required,
* is constexpr,
* doesn't change the rank of the extent.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/extents/class_mandates_neg.cc: New test.
	* testsuite/23_containers/mdspan/extents/ctor_copy.cc: New test.
	* testsuite/23_containers/mdspan/extents/ctor_ints.cc: New test.
	* testsuite/23_containers/mdspan/extents/ctor_shape.cc: New test.
	* testsuite/23_containers/mdspan/extents/custom_integer.cc: New test.
	* testsuite/23_containers/mdspan/extents/misc.cc: New test.

Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>