Update format of junit xml output to mark failures as such

[glenjamin]

As noted in #557 (comment) - the current Junit xml output treats each lint issue as a passing test.

This PR expands the details and shuffles things around so that it shows up as a failing test instead.

There didn't seem to be any automated tests for this functionality, so I haven't added any new ones.

Here's a sample of what that looks like with CircleCI's junit parsing
https://circleci.com/gh/glenjamin/golangci-lint/5

I don't know which other popular tools do junit parsing like this - I based this implementation loosely off https://github.com/eslint/eslint/blob/master/lib/cli-engine/formatters/junit.js

[CLAassistant]

All committers have signed the CLA.

[glenjamin]

I've inspected the SVG, and I can't see any difference - I'm not sure why this would have changed on my branch.

[Helcaraxan]

@glenjamin - See #625 for the solution. Once that fix is merged all latests PR can be rebased on top of the fresh master and go green.

[jirfag]

thank you!