Define 'FocusTest' debugging prefix to prevent all other test cases from running

[mdwhatcott]

Currently, to run a single test you have to skip all other tests:

func (this *MyFixture) TestA() {}
func (this *MyFixture) SkipTestB() {}
func (this *MyFixture) SkipTestC() {}

What I'd like to enable is a simple marker on the desired test:

func (this *MyFixture) FocusTestA() {}
func (this *MyFixture) TestB() {}
func (this *MyFixture) TestC() {}

Of course, this would be a non-issue if we had more full-fledged support from Goland, but this would also allow users of other IDEs to accomplish this behavior.