Don't show test lenses in libraries outside of the workspace

[lnicola]

    So I just ran into this issue with types implemented in the standard library.

Here are my repro steps:

1. Inside src/main.rs include the following:

use std::sync::Mutex;

fn main() {
    let m = Mutex::new(0);
}

2. Place your cursor (|) in Mutex::ne|w and trigger "Go to definition" (A.K.A. F12).
3. VS Code will display the code lens option to "Run Doctest" in the library's definition:

4. After clicking on that action, the output is:

Now, here are some thoughts. Note that I'm a Rust beginner (I encountered this while going though the Rust book hehe) so please take it with a grain of salt <3.

1. I don't think that code lens inside code that I didn't write myself should be enabled at all, since those feel like "implementor/dev-only" tools, and I don't immediately see why the clients of the APIs would need them. So I wouldn't be upset if this feature is removed (it's not working anyway...).
2. From the picture included in point 4 above, it seems like cargo test is running inside my current workspace, and not where the code lens is being displayed.

Originally posted by @MariaSolOs in #8248 (comment)

[MariaSolOs]

Could I take this @lnicola? :)

[MariaSolOs]

Also, do we want to just disable test-related lenses or all code lenses for external libraries?

[Veykril]

Only tests I would say, other code lenses do make sense in external libraries still

[lnicola]

Hmm. In an ideal world, we could have rlibs for the libraries, so things like "find all references" wouldn't work. I wouldn't mind if we didn't look at the function bodies in external crates.

[Veykril]

Right, find all references isn't too useful there (I keep forgetting that's a thing as i have that turned off), but I think the implementations lenses would still make sense?

[MariaSolOs]

Alright, I have a fix in #13686 :)

[lnicola]

Fixed in #13686