internal: make xtask lighter

Moving tests to `rust-analyzer` crate allows removing walkdir dependency
from `xtask`. It does seem more reasonable to keep tidy tests outside of
the "build system" and closer to other integration tests.

xtask/src/main.rs

@@ -9,9 +9,6 @@
 //! `.cargo/config`.
 mod flags;
 
-#[cfg(test)]
-mod tidy;
-
 mod install;
 mod release;
 mod dist;