libstdc++: Prepare test code for default_accessor for reuse.

libstdc++: Prepare test code for default_accessor for reuse.

All test code of default_accessor can be reused. This commit moves
the reuseable code into a file generic.cc and prepares the tests for
reuse with aligned_accessor.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/accessors/default.cc: Delete.
	* testsuite/23_containers/mdspan/accessors/generic.cc: Slightly
	generalize the test code previously in default.cc.

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