Breaking changes to the test interface for AVA 4

[novemberborn]

See #2435. This will improve context typing and simplify usage.

• Add test.macro(exec) and test.macro({exec, title}). This will return a test.Macro instance which can be passed to test()
• Remove ability to pass an array of macros or implementations to test()
• Fix the typing of test implementations (or macro executors) to allow promise-returning, observable returning and any-returning functions

[novemberborn]

• Fix the typing of test implementations (or macro executors) to allow promise-returning, observable returning and any-returning functions

This isn't really an issue since the fallback type is void, but having the PromiseLike and Subscribable types does help if you're ever tempted to write these by hand, within the implementation…