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.

lib/filetests/src/test_cat.rs

@@ -0,0 +1,37 @@
+//! The `cat` subtest.
+
+use std::borrow::Cow;
+use cretonne::ir::Function;
+use cton_reader::TestCommand;
+use subtest::{self, SubTest, Context, Result as STResult};
+
+/// Object implementing the `test cat` sub-test.
+///
+/// This command is used for testing the parser and function printer. It simply parses a function
+/// and prints it out again.
+///
+/// The result is verified by filecheck.
+struct TestCat;
+
+pub fn subtest(parsed: &TestCommand) -> STResult<Box<SubTest>> {
+    assert_eq!(parsed.command, "cat");
+    if !parsed.options.is_empty() {
+        Err(format!("No options allowed on {}", parsed))
+    } else {
+        Ok(Box::new(TestCat))
+    }
+}
+
+impl SubTest for TestCat {
+    fn name(&self) -> Cow<str> {
+        Cow::from("cat")
+    }
+
+    fn needs_verifier(&self) -> bool {
+        false
+    }
+
+    fn run(&self, func: Cow<Function>, context: &Context) -> STResult<()> {
+        subtest::run_filecheck(&func.display(context.isa).to_string(), context)
+    }
+}