[libc++] [test] Refactor range.prim/empty.pass.cpp.

No decrease in test coverage intended. The original goal here
was just to get rid of the global name `sentinel` so that we can
rename the `sentinel_wrapper` in "test_iterators.h" to `sentinel`;
but then I took a closer look at the offending tests and saw
that some of them probably weren't testing what they intended.

Also, add one `/*explicit*/` and one #if'ed out test indicating
bugs in the current ranges::empty (to be fixed by D115312 or
some equivalent patch).

Reviewed as part of D115272.