[libc++][test] Fix iterator assertion in span.cons/deduct.pass.cpp

Two tests in span.cons/deduct.pass.cpp accidentally check whether the
iterator range from member begin and member end are equivalent to the
ones from free begin and free end. This is obviously true and not
intended. Correct the intent by comparing the size/data from the span
with the source input.

While in the neighborhood, add test for const int arr[N], remove extraneous
type aliases, unused <type_traits> header, and the
disable_missing_braces_warning.h include.

Reviewed By: Quuxplusone, ldionne, #libc

Differential Revision: https://reviews.llvm.org/D109668