Run all tests in the module

[matklad]

We have a feature to invoke the right cargo test command for the test at caret:

(In VS Code, it is triggered by the Rust Run action)

Currently, it works only for #[test] functions, but it would be useful to run test modules as well.

Here's how the feature works currently:

runnables function discovers #[test] functions:
https://github.com/rust-analyzer/rust-analyzer/blob/0043d7e9c7f27af95cbf299d5b92ce28ba9945ab/crates/ra_editor/src/lib.rs#L119-L140

The function is exposed on Analysis struct (the API between language server and "compiler") here:
https://github.com/rust-analyzer/rust-analyzer/blob/0043d7e9c7f27af95cbf299d5b92ce28ba9945ab/crates/ra_analysis/src/lib.rs#L245-L248

in main_loop/handlers we add information about current Cargo target/package to it and handle, as JSON, to the editor:

https://github.com/rust-analyzer/rust-analyzer/blob/0043d7e9c7f27af95cbf299d5b92ce28ba9945ab/crates/ra_lsp_server/src/main_loop/handlers.rs#L255-L268

To support testing modules, we need to add fully-qualified module name to the command line. The steps to do so would be

• move the runnable function from ra_editor to ra_analysis: ra_editor knows nothing about module
• in the runnable function, detect not only functinons, but mods with test functions as well
• make a function to compute a foo::bar::baz path to a module. See the implementation of crate_root function, which contains "traverse parent modules" logic.
• add module path to a Runnable struct
• adjust the handle_runnables function to construct a command line, using the full path