Fix tests that fail without PCH

Fix tests that fail without PCH

The recent change to stop transitively including <string> broke some
tests, but only when the library is configured without PCH, because
otherwise the <string> header still gets included via the precompiled
<bits/stdc++.h> header.

	* testsuite/20_util/bad_function_call/what.cc: Include <string> header
	for std::string.
	* testsuite/20_util/shared_ptr/cons/weak_ptr_expired.cc: Likewise.
	* testsuite/20_util/tuple/cons/allocator_with_any.cc: Include <memory>
	header for std::allocator.
	* testsuite/23_containers/array/tuple_interface/tuple_element.cc: Add
	using-declaration for std::size_t.
	* testsuite/23_containers/array/tuple_interface/tuple_size.cc:
	Likewise.
	* testsuite/23_containers/deque/cons/55977.cc: Include <istream> for
	std::istream.
	* testsuite/23_containers/vector/cons/55977.cc: Likewise.
	* testsuite/experimental/map/erasure.cc: Include <string> for
	std::string.
	* testsuite/experimental/unordered_map/erasure.cc: Likewise.

From-SVN: r272376