feat: added support for types assignable to special step argument types

[bondsb]

This update adds support for injecting special step arguments whose types are assignable to the 4 supported types (in contrast to supporting only arguments whose types are precisely those 4).

Background use case: Terratest

I wanted to use gocuke with Terratest. Injecting a gocuke.TestingT into my suite didn't provide the Name() method required by Terratest:

./gocuke_test.go:51:25: cannot use s (variable of type *suite) as "github.com/gruntwork-io/terratest/modules/testing".TestingT value in argument to terraform.InitAndApply: *suite does not implement "github.com/gruntwork-io/terratest/modules/testing".TestingT (missing method Name)

I added Name() to a custom interface that includes all the methods in gocuke.TestingT. But this was not injected via the special step arguments.

My solution was to modify this behavior in runner.go to inject based on assignability instead of equality.

[aaronc]

I think this is okay but can we add some small test case?

[bondsb]

Added a copy of the simple test that fails with the previous code and passes with the change.