Test: fuzzers: replace cib_file_fuzzer test

Fuzzing cib_file_read_and_verify() found only memory leaks internal to
libxml2, which is unhelpful. Also, fuzzing that function should have a
custom input generator rather than random data interpreted as XML.

Fuzz cib_file_new() instead, which is entirely under our control.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69436
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69440
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69495