Added assert-cli; simple test case for --write-mode=check

Author: davidbarsky

Cargo.lock

@@ -50,6 +50,7 @@ rustc-ap-syntax = "67.0.0"
 
 [dev-dependencies]
 lazy_static = "1.0.0"
+assert_cli = "0.5"
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2.11"