Move the filetest harness into its own crate.

This allows us to run the tests via a library call rather than just
as a command execution. And, it's a step toward a broader goal, which
is to keep the code in the top-level src directory minimal, with
important functionality exposed as crates.

cranelift/src/rsfilecheck.rs

@@ -1,3 +1,7 @@
+//! The `filecheck` sub-command.
+//!
+//! This file is named to avoid a name collision with the filecheck crate.
+
 use CommandResult;
 use utils::read_to_string;
 use filecheck::{CheckerBuilder, Checker, NO_VARIABLES};