libctf: fix testsuite bugs revealed by -Wall

Most of these are harmless, but some of the type confusions and especially
a missing ctf_strerror() on an error path were actual bugs that could
have resulted in test failures crashing rather than printing an error
message.

libctf/
	* testsuite/libctf-lookup/enumerator-iteration.c: Fix type
        confusion, signedness confusion and a missing ctf_errmsg().
	* testsuite/libctf-regression/libctf-repeat-cu-main.c: Return 0 from
        the test function.
	* testsuite/libctf-regression/open-error-free.c: Fix signedness
        confusion.
	* testsuite/libctf-regression/zrewrite.c: Remove unused label.