[libc++] Remove uses of printf in some test support headers

In the test suite, we generally don't use printf or other reporting
utilities. It's not that it wouldn't be useful, it's just that some
platforms don't support IO.

Instead, we try to keep test cases small and self-contained so that
we can reasonably easily reproduce failures locally and debug them.
This patch removes printf in some of the last places in the test suite
that used it. The only remaining places are in a deque test and in the
filesystem tests. The filesystem tests are arguably fine to keep using
IO, since we're testing <filesystem>. The deque test will be handled
separately.

Differential Revision: https://reviews.llvm.org/D114282