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.

libstdc++-v3/testsuite/24_iterators/move_iterator/p2520r0.cc

@@ -1,5 +1,6 @@
 // { dg-options "-std=gnu++20" }
 // { dg-do compile { target c++20 } }
+// { dg-add-options no_pch }
 
 // Verify P2520R0 changes to move_iterator's iterator_concept, which we treat
 // as a DR against C++20.

libstdc++-v3/testsuite/std/format/functions/format.cc

@@ -1,5 +1,6 @@
 // { dg-options "-std=gnu++20" }
 // { dg-do run { target c++20 } }
+// { dg-add-options no_pch }
 
 #include <format>
 

libstdc++-v3/testsuite/std/format/functions/format_c++23.cc

@@ -1,4 +1,5 @@
 // { dg-do run { target c++23 } }
+// { dg-add-options no_pch }
 // This test does not have -std=gnu++20 in dg-options so that format.cc
 // can be tested for e.g. -std=c++26
 #include "format.cc"