Exactly same project, renaming 
Renaming app.rs -> main.rs:

/Users/bRad/.cargo/bin/cargo test --color=always
   Compiling xargo-lld-wrapper v0.1.0 (file:///Users/bRad/Development/bg/experiments/rust/rpi3/xargo-lld-wrapper)
error: cannot declare a new module at this location
  --> src/lib.rs:28:5
   |
28 | mod tests;
   |     ^^^^^
   |
note: maybe move this module `src` to its own directory via `src/mod.rs`
  --> src/lib.rs:28:5
   |
28 | mod tests;
   |     ^^^^^
note: ... or maybe `use` the module `tests` instead of possibly redeclaring it
  --> src/lib.rs:28:5
   |
28 | mod tests;
   |     ^^^^^

error: aborting due to previous error

error: Could not compile `xargo-lld-wrapper`.

To learn more, run the command again with --verbose.

Process finished with exit code 101