tests/fuzzers/abi: better test generation

[MariusVanDerWijden]

Fixes the fuzzers

2021/01/11 11:12:10 workers: 16, corpus: 212 (24s ago), crashers: 0, restarts: 1/9679, execs: 7085542 (12365/sec), cover: 1996, uptime: 9m33s
2021/01/11 11:12:13 workers: 16, corpus: 212 (27s ago), crashers: 0, restarts: 1/9663, execs: 7121919 (12364/sec), cover: 1996, uptime: 9m36s
2021/01/11 11:12:16 workers: 16, corpus: 212 (30s ago), crashers: 0, restarts: 1/9699, execs: 7158058 (12363/sec), cover: 1996, uptime: 9m39s
2021/01/11 11:12:19 workers: 16, corpus: 212 (33s ago), crashers: 0, restarts: 1/9669, execs: 7194322 (12361/sec), cover: 1996, uptime: 9m42s

It decreases a bit the ability of the fuzzers to test everything, since the Pack(Unpack(in)) == in test was removed. However this test would often fail if the ABI only had a single argument, eg. 0x123...890 would be unpacked to 0x0000...0090 if the ABI only specifies a single uint8 argument, therefore the packing step could never recreate in.

The PR also reenables the abi fuzzer for oss-fuzz (it was disabled since it created so many errors)
@holiman If you think it is to early to put it on oss-fuzz we can also drop the last commit ed4865f

[holiman]

LGTM. We'll notice if it starts failing on oss-fuzz, it's not a biggie to fix it later if so