test-src in elf/Makefile

[Roland McGrath]

> What is this supposed to fix:
> 
> 	* elf/Makefile (test-srcs): Add $(modules-names) to it.
> 
> test-srcs is supposed to only contain targets for binaries, but
> modules-names only contains .so targets.

This was the easy way to make sure that cppflags-iterator.mk got used for
compiling those modules, which should have -DNOT_IN_libc.  I now notice
that test-srcs is used for binaries-bounded, which is not appropriate for
these modules, but I am not sure it hurts.