gcs: Overhaul tests and benchmarks.

[davecgh]

This requires PRs #1843 and #1844.

This rewrites the tests to make them more consistent with the rest of the code base and significantly increases their coverage of the code.

It also reworks the benchmarks to actually benchmark what their names claim, renames them for consistency, and make them more stable by ensuring the same prng seed is used each run to eliminate variance introduced by different values.

Finally, it removes an impossible to hit condition from the bit reader and adds a couple of additional checks to harden the filters against potential misuse.

This is part of the ongoing process to cleanup and improve the gcs module to the quality level required by consensus code for ultimate inclusion in header commitments.

[davecgh]

$ go test -coverprofile=cov.out && go tool cover -func=cov.out
PASS
coverage: 93.8% of statements
ok      github.com/decred/dcrd/gcs/v2   0.422s
github.com/decred/dcrd/gcs/v2/bits.go:18:       writeOne                100.0%
github.com/decred/dcrd/gcs/v2/bits.go:31:       writeZero               100.0%
github.com/decred/dcrd/gcs/v2/bits.go:44:       writeNBits              95.7%
github.com/decred/dcrd/gcs/v2/bits.go:93:       newBitReader            100.0%
github.com/decred/dcrd/gcs/v2/bits.go:102:      readUnary               100.0%
github.com/decred/dcrd/gcs/v2/bits.go:126:      readNBits               97.1%
github.com/decred/dcrd/gcs/v2/gcs.go:43:        Len                     100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:44:        Less                    100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:45:        Swap                    100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:64:        NewFilter               96.9%
github.com/decred/dcrd/gcs/v2/gcs.go:141:       FromBytes               100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:163:       FromNBytes              100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:182:       Bytes                   100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:191:       NBytes                  100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:197:       P                       100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:202:       N                       100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:208:       Match                   100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:250:       MatchAny                100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:314:       readFullUint64          85.7%
github.com/decred/dcrd/gcs/v2/gcs.go:330:       Hash                    100.0%
github.com/decred/dcrd/gcs/v2/gcs.go:346:       MakeHeaderForFilter     0.0%
total:                                          (statements)            93.8%

Note that no coverage was added for MakeHeaderForFilter because it will removed in upcoming commits.