[libc++][format] Improves the handle test.

A formatter using a handle only needs to satisfy the BasicFormatter
requirements. The current test allowed more than that minimum. Changed
it to the minimum to make sure it works.

This was due to a post-commit review comment of @vitaut in D121530.

Reviewed By: ldionne, vitaut, #libc

Differential Revision: https://reviews.llvm.org/D127767