Ability to write tests right in the same file

[MahdiBM]

Description

It would be nice if we had the ability to write test in the same file, and not in files far away in a dedicated Tests folder.
This will have some benefits like:

• The code and the tests are just simply closer apart.
• We could use access modifiers like fileprivate to test parts of the code in the same file, instead of having to workaround this by @testable importing which makes users have to not use private access modifiers just to be able to test a function. Or using stuff like @_spi.

Languages like Rust do support this.

I saw some discussions about this a few months ago, I think the compiler needs to support better discovery of stuff like tests for this to be possible.

[grynspan]

Tracked internally as rdar://112339873.