Tests in fuzz directory

[sanket1729]

I guess the purpose of duplicate_crash() functions to replicate crashes on Travis for debugging, but I don't think they should belong in the fuzz directory.
The current codebase seems to have the same fuzz test extend_from_vec across all files.

[apoelstra]

Right - duplicate_crash isn't there for regression tests. (These should go in the main tree, not the fuzz/ directory.) It's just a convenience function for copy/pasting the output of a failed fuzztest so you can check whether a fix works.

Agree that's a bit wasteful that extend_from_vec is duplicated across every single fuzztest. We could factor it out. Even more wastefully, it is duplicated in every fuzztest in rust-bitcoin and rust-secp and bitcoin_hashes as well :)

@sanket1729 can you PR to bring duplicate_crash_2 out of fuzz/ to somewhere else?

[kiminuo]

@apoelstra @sanket1729 What about #96?