test(dht): add test for DhtRequest encoding/decoding

[kurnevsky]

TODO:

• https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L200
• https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L208
• https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L212
• https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L314
• https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L322
• https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L326
• remove old tests like https://coveralls.io/builds/15490518/source?filename=src%2Ftoxcore%2Fdht_new%2Fpacket.rs#L869

[coveralls]

Pull Request Test Coverage Report for Build 169

• 127 of 130 (97.69%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+1.07%) to 97.082%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/toxcore/dht_new/packet.rs	127	130	97.69%

Files with Coverage Reduction	New Missed Lines	%
src/toxcore/dht_new/packet.rs	2	98.42%

Totals
Change from base Build 154:	1.07%
Covered Lines:	6254
Relevant Lines:	6442

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💛 - Coveralls