Fixes #404: Use log.rs and a file-per-thread logger instead of the dbg! macro;

lib/filetests/src/lib.rs

@@ -18,11 +18,13 @@
     )
 )]
 
-#[macro_use(dbg)]
 extern crate cranelift_codegen;
+extern crate file_per_thread_logger;
 extern crate cranelift_reader;
 extern crate filecheck;
 extern crate num_cpus;
+#[macro_use]
+extern crate log;
 
 use cranelift_reader::TestCommand;
 use runner::TestRunner;