Fix docker machine executor test filename

What does this MR do?

Fixes the wrongly named test.go file in the machine executor package.

Why was this MR needed?

As reported by @tmaczukin in !1902 (comment 303508606) I had wrongly renamed the file to test.go which lead to it not being executed in tests. This brings the files to the convention of other executors -> executors/<executor_name>/<executor_name>.go

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?