[libc++][NFC] Centralize test for support of == and != in ranges (#78481)

Previously, tests for whether comparison using == was supported by
iterators derived from ranges adaptors was spread throughout the testing
codebase. This PR centralizes the implementation of those tests.

GitOrigin-RevId: 808d794a45e169601ff16f72beae2f7bd79342a2
11 files changed