libstdc++: Add tests for layout_stride.

libstdc++: Add tests for layout_stride.

Implements the tests for layout_stride and for the features of the other
two layouts that depend on layout_stride.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/layouts/class_mandate_neg.cc: Add
	tests for layout_stride.
	* testsuite/23_containers/mdspan/layouts/ctors.cc: Add test for
	layout_stride and the interaction with other layouts.
	* testsuite/23_containers/mdspan/layouts/empty.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/mapping.cc: Ditto.
	* testsuite/23_containers/mdspan/layouts/stride.cc: New test.

Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>