multi: add getwork tests.

[dnldd]

This adds rpc handler tests for getwork.

Work towards #2069.

[davecgh]

The interface pieces look good now, but I see that the test coverage is missing several paths and submission altogether.

$ go test -run=TestHandleGetWork -coverprofile=cov.out
$ go tool cover -func=cov.out | grep -E handleGetWork
github.com/decred/dcrd/internal/rpcserver/rpcserver.go:3290:            handleGetWorkRequest            92.7%
github.com/decred/dcrd/internal/rpcserver/rpcserver.go:3394:            handleGetWorkSubmission         0.0%
github.com/decred/dcrd/internal/rpcserver/rpcserver.go:3482:            handleGetWork                   93.3%

[dnldd]

All paths not covered could not be hit.

go tool cover -func=cov.out | grep -E handleGetWork
github.com/decred/dcrd/internal/rpcserver/rpcserver.go:3287:               handleGetWorkRequest            95.1%
github.com/decred/dcrd/internal/rpcserver/rpcserver.go:3391:               handleGetWorkSubmission         92.9%
github.com/decred/dcrd/internal/rpcserver/rpcserver.go:3479:               handleGetWork                   100.0%

Based on that I'd suggest removing the error check here since CheckProofOfWork only returns standalone.RuleError types.

[davecgh]

Regarding your comment about removing the unexpected error type check, I agree that it's currently impossible, and hence can't be tested, but I also know that code gets refactored and thus assumptions that like always end up broken over time.

As a case in point, I see you already caught a case where it was checking for the wrong type of error. In this path, if the code gets refactored and some other issue happens, it would be completely hidden if the error check is removed as it would just look a miner submitting a block that didn't meet the work requirements even though it could be a real issue.

[rstaudt2]

Looks good. Super minor but I did notice that there is a typo in the commit message (This fixes a proof of work error check and adds rpc handler tests for getWork.)