Fix intermittent build error

The fmt test will cause clippy dev to be compiled and run. The clippy dev
dependencies are currently stored at `target/debug/deps` when this happens.
This location sometimes causes conflicts with ui tests which result in the
build error "thread 'compile_test' panicked at 'Found multiple rlibs for crate
`regex` ...".

This commit forces the clippy_dev dependencies to be stored under
`clippy_dev/target/` which seems to resolve this problem.

Author: Michael Wright

.cargo/config

@@ -1,6 +1,6 @@
 [alias]
 uitest = "test --test compile-test"
-dev = "run --package clippy_dev --bin clippy_dev --manifest-path clippy_dev/Cargo.toml --"
+dev = "run --target-dir clippy_dev/target --package clippy_dev --bin clippy_dev --manifest-path clippy_dev/Cargo.toml --"
 
 [build]
 rustflags = ["-Zunstable-options"]