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 <karljohan-alm@garage.co.jp>

.gitignore

@@ -7,8 +7,9 @@ src/bitcoind
 src/bitcoin-cli
 src/bitcoin-tx
 src/bitcoin-wallet
+src/test/fuzz
+!src/test/fuzz/*.*
 src/test/test_bitcoin
-src/test/test_bitcoin_fuzzy
 src/qt/test/test_bitcoin-qt
 
 # autoreconf