[libc++] Fix filesystem tests on platforms that don't have IO

This patch moves a few tests that were still using std::fprintf to
using TEST_REQUIRE instead, which provides a single point to tweak
for platforms that don't implement fprintf. As a fly-by fix, it also
avoids including `time_utils.h` in filesystem_clock.cpp when it is
not required, since that header makes some pretty large assumptions
about the platform it is on.

Differential Revision: https://reviews.llvm.org/D155019