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.

libctf/testsuite/libctf-regression/libctf-repeat-cu-main.c

@@ -2,4 +2,4 @@ typedef short ret_t;
 int a (void);
 int b (void);
 int c (void);
-int blah (void) { a(); b(); c(); }
+int blah (void) { a(); b(); c(); return 0; }

libctf/testsuite/libctf-regression/open-error-free.c

@@ -143,7 +143,7 @@ int main (void)
      failure after we corrupt it: the leak is only observable if the dict gets
      malloced, which only happens after that point.)  */
 
-  if ((written = ctf_write_mem (fp, &written_size, (size_t) -1)) == NULL)
+  if ((written = (char *) ctf_write_mem (fp, &written_size, (size_t) -1)) == NULL)
     goto write_err;
 
   ctf_dict_close (fp);

libctf/testsuite/libctf-regression/zrewrite.c

@@ -150,7 +150,4 @@ main (int argc, char *argv[])
  write_stderr:
   fprintf (stderr, "%s: cannot open for writing: %s\n", argv[0], strerror (errno));
   return 1;
- read_err: 
-  fprintf (stderr, "%s: cannot read\n", argv[0]);
-  return 1;
 }