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/tests/filetests.rs

@@ -0,0 +1,7 @@
+extern crate cton_filetests;
+
+#[test]
+fn filetests() {
+    // Run all the filetests in the following directories.
+    cton_filetests::run(false, vec!["filetests".into(), "docs".into()]).expect("test harness");
+}