[libc++] [ranges] Simplify our idiom for testing niebloid-ness.

[libc++] [ranges] Simplify our idiom for testing niebloid-ness.

In the test files, replace the old-style tests with a simple static_assert,
matching the current style as depicted in e.g.
`ranges_uninitialized_default_construct.pass.cpp`.

Preserve `is_function_like` (but renamed to `is_niebloid`) at
ldionne's request. The removal of this test helper will happen
in D116570 if at all.

Differential Revision: https://reviews.llvm.org/D116384