c++: xtreme-header modules tests cleanups

c++: xtreme-header modules tests cleanups

This adds some recently implemented C++20/23 library headers to the
xtreme-header tests as appropriate.  Also, it looks like we can safely
re-add <execution> and remove the NO_ASSOCIATED_LAMBDA workaround.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/xtreme-header-2.h: Include <execution>.
	* g++.dg/modules/xtreme-header-6.h: Include implemented
	C++20 library headers.
	* g++.dg/modules/xtreme-header.h: Likewise.  Remove
	NO_ASSOCIATED_LAMBDA workaround.  Include implemented C++23
	library headers.