[libc++][test] Verify std::views::drop and std::views::join are CPOs

[jloser]

Summary

std::views::drops and std::views::join have been implemented, but the tests
verifying the CPOs for them are been implemented is still commented out.
Uncomment them.

[philnik]

LGTM assuming CI passes.

libcxx/test/std/library/description/conventions/customization.point.object/cpo.compile.pass.cpp

constexpr bool test(CPO& o, Args&&...) {
   static_assert(std::invocable<const T&, Args...>);
   static_assert(std::invocable<T, Args...>);
   static_assert(std::invocable<const T, Args...>);
 
   return true;
 }
 
 int a[10];
-//int arrays[10][10];
+int arrays[10][10];
 //std::pair<int, int> pairs[10];
 
 // [concept.swappable]
 static_assert(test(std::ranges::swap, a, a));
 
 // [iterator.cust]
 static_assert(test(std::ranges::iter_move, a + 0));
 static_assert(test(std::ranges::iter_swap, a + 0, a + 1));

 static_assert(test(std::views::iota, 1, 10));
 //static_assert(test(std::views::istream<int>, 1);
 static_assert(test(std::views::single, 4));
 
 // [range.adaptors]
 static_assert(test(std::views::all, a));
 static_assert(test(std::views::common, a));
 static_assert(test(std::views::counted, a, 10));
-//static_assert(test(std::views::drop, a, 10));
+static_assert(test(std::views::drop, a, 10));
 //static_assert(test(std::views::drop_while, a, [](int x){ return x < 10; }));
 //static_assert(test(std::views::elements<0>, pairs));
 static_assert(test(std::views::filter, a, [](int x){ return x < 10; }));
-//static_assert(test(std::views::join, arrays));
+static_assert(test(std::views::join, arrays));
 //static_assert(test(std::views::split, a, 4));
 static_assert(test(std::views::lazy_split, a, 4));
 static_assert(test(std::views::reverse, a));
 static_assert(test(std::views::take, a, 10));
 //static_assert(test(std::views::take_while, a, [](int x){ return x < 10; }));
 static_assert(test(std::views::transform, a, [](int x){ return x + 1; }));