Add fuzzer to check round-tripping

[mgeisler]

Hi @BrettDong, thanks for maintaining the library. It's been very useful for me in mdbook-i18n-helpers!

Would you be up for adding some fuzzers to the library? I ran into an issue where I accidentally generated a .pot file with a update msgid "" entry: google/mdbook-i18n-helpers#64. I'm thinking to add fuzzers to guard against this: google/mdbook-i18n-helpers#65.

The fuzzer I have in mind would essentially use the public polib API to generate a .pot file and then check that it can be loaded — thereby checking that polib can load all files it generates. After loading the generated file, the fuzzer would check that the entries are identical to what was saved. This way we can be use that the library preserves the information.