Add gunit support (test runner)

[mdwhatcott]

https://github.com/smartystreets/gunit

Similar to #1976.

[mdwhatcott]

Since gunit is basically just a sub-test generator that sits on top of go test and the standard library testing package, here is the only missing functionality from the regular "gotest" run configuration:

• The green 'run' arrow that appears in the line-number gutter needs to be wired up such that you can invoke a single test method (resulting in something like go test -run 'fixture-struct-name/method-name')
• Test cases listed in results window should be more interactive and allow you to navigate back to the test method and/or invoke just that test case.

It doesn't seem like a lot would have to be done to make it happen but I'm not sure.

smarty/gunit#12