libstdc++: Remove redundant parens in mdspan testsuite.

libstdc++: Remove redundant parens in mdspan testsuite.

A recent commit improved the macro VERIFY to eliminate the need for
certain parens. This commit updates the test code in

  23_containers/mdspan

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/mdspan/extents/ctor_ints.cc: Remove
	superfluous parens.
	* testsuite/23_containers/mdspan/extents/ctor_shape.cc: Ditto.
	* testsuite/23_containers/mdspan/mdspan.cc: Ditto.

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