`<ranges>`: Add missing test coverage for `ranges::view_interface::c(begin|end)`

#3187 introduced new members to ranges::view_interface: cbegin and cend. While there is small coverage for those functions in P0896R4_ranges_subrange/test.compile.pass.cpp, I believe that we should add extra assertions to each views tests (see #3350 for example).

• as_const (#3920),
• as_rvalue,
• adjacent (#3508),
• adjacent_transform (#3546),
• cartesian_product (#3561),
• chunk,
• chunk_by,
• common (#3612),
• drop (#3390),
• drop_while (#3390),
• elements (#3350),
• empty (#3553),
• enumerate (#3472),
• filter (#3612),
• std::generator,
• iota (#3553),
• istream (#3553),
• join (#3612),
• join_with,
• lazy_split,
• owning_view (#3753),
• ref_view (#3612),
• repeat (#3553),
• reverse (#3612),
• single (#3553),
• slide,
• split,
• stride (#3920),
• subrange (#3612),
• take (#3390),
• take_while (#3390),
• transform (#3612),
• zip (#3920),
• zip_transform (#3322).

The existing coverage is in P0896R4_ranges_subrange/test.compile.pass.cpp; compare it with the views tests and the example from #3350. Add equivalent cbegin/cend assertions for the unchecked entries—as_rvalue, chunk, chunk_by, generator, join_with, lazy_split, slide, and split—then run the affected tests; done means each unchecked view is covered.