gitignore: ignore fuzz binaries, remove test_bitcoin_fuzzy
gitignore: ignore fuzz binaries, remove test_bitcoin_fuzzy

The fuzzing gitignores haven't been updated since a4153e2
in 2016 that added an initial simple fuzzing framework.

This commit:

- removes `src/test/test_bitcoin_fuzzy` which is no longer
  used in favor of `src/test/fuzz`

- ignores the src/test/fuzz directory, then un-ignores any
  files in it with an extension

Co-authored-by: Karl-Johan Alm <9b4559154a893a04b0dd6be23e5b5444f7855099@garage.co.jp>

DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin