libctf: tests: prune warnings from compiler output

We were failing to call prune_warnings appropriately, leading to
false-positive test failures on some platforms (observed on
sparclinux).

libctf/ChangeLog:

	* testsuite/lib/ctf-lib.exp: Prune warnings from compiler and
	linker output.
	* testsuite/libctf-regression/libctf-repeat-cu.exp: Likewise,
	and ar output too.