Add coverage tools to Makefile

[codysoyland]

Overview

make cover-viz will now generate an HTML coverage report for the entire codebase.

Pull request checklist

• I have read the contributing guide.
• I have agreed to the Contributor License Agreement.
• I have updated the documentation.
• I have resolved any merge conflicts.
• I have included tests that cover my changes.
• All new and existing tests pass.

Code review checklist

This is the checklist that the reviewer will follow while reviewing your pull request. You do not need to do anything with this checklist, but be aware of what the reviewer will be looking for.

• Ensure that any changes to external docs have been included in this pull request.
• If the changes require that minor/major versions need to be updated, tag the PR appropriately.
• Ensure the new code is properly commented and follows Idiomatic Go.
• Check that tests have been written and that they cover the new functionality.
• Run tests and ensure they pass.
• Build and run the code, performing any applicable integration testing.

[yuce]

Why do we run separate coverage for subpackages? Is there any difference from running it in the root?

[codysoyland]

There does not appear to be a way to generate a coverage report for multiple packages with a single invocation of go test:

$ go test -coverprofile=coverage.out github.com/pilosa/pilosa github.com/pilosa/pilosa/cmd
cannot use test profile flag with multiple packages

[yuce]

I've tried make cover and make cover-viz and they work!