libstdc++: Disable PCH for tests that rely on include order

These tests expect to be able to #undef a feature test macro and then
include <version> to get it redefined. But if <version> has already been
included by the <bits/stdc++.h> PCH then including it again does nothing
and the macro remains undefined.

libstdc++-v3/ChangeLog:

	* testsuite/24_iterators/move_iterator/p2520r0.cc: Add no_pch.
	* testsuite/std/format/functions/format.cc: Likewise.
	* testsuite/std/format/functions/format_c++23.cc: Likewise.