Add marker interfaces for SetUpTest and TearDownTest methods

[rillig]

Right now, when I add a SetUpTest method to my test suite, JetBrains GoLand reports the parameter as unused, as it doesn't know about these test fixtures.

One way to fix this would be to teach GoLand the API of check.v1.

Another way would be to add the following marker interfaces to check.v1:

type SetUpFixture interface {
	SetUpTest(*check.C)
}
type TearDownFixture interface {
	TearDownTest(*check.C)
}

When this is done, GoLand sees that my SetUpTest method implements this interface, and it no longer complains about the seemingly unused parameter. My guess is that other IDEs behave similar. This would also mean that my SetUpTest method would be marked as implementing this interface, therefore it would be a just a single click to the documentation of the test fixtures:

The gutter icons give me a clue that I spelled the method names correctly, which is valuable feedback.