Add tests for setcontext on the context from makecontext

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

	* stdlib/Makefile ((tests): Add tst-setcontext6, tst-setcontext7,
	tst-setcontext8 and tst-setcontext9.
	* stdlib/tst-setcontext6.c: New file.
	* stdlib/tst-setcontext7.c: Likewise.
	* stdlib/tst-setcontext8.c: Likewise.
	* stdlib/tst-setcontext9.c: Likewise.