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.