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_preopt.rs

@@ -0,0 +1,50 @@
+//! Test command for testing the preopt pass.
+//!
+//! The resulting function is sent to `filecheck`.
+
+use cretonne::ir::Function;
+use cretonne;
+use cretonne::print_errors::pretty_error;
+use cton_reader::TestCommand;
+use subtest::{SubTest, Context, Result, run_filecheck};
+use std::borrow::Cow;
+use std::fmt::Write;
+
+struct TestPreopt;
+
+pub fn subtest(parsed: &TestCommand) -> Result<Box<SubTest>> {
+    assert_eq!(parsed.command, "preopt");
+    if !parsed.options.is_empty() {
+        Err(format!("No options allowed on {}", parsed))
+    } else {
+        Ok(Box::new(TestPreopt))
+    }
+}
+
+impl SubTest for TestPreopt {
+    fn name(&self) -> Cow<str> {
+        Cow::from("preopt")
+    }
+
+    fn is_mutating(&self) -> bool {
+        true
+    }
+
+    fn run(&self, func: Cow<Function>, context: &Context) -> Result<()> {
+        // Create a compilation context, and drop in the function.
+        let mut comp_ctx = cretonne::Context::new();
+        comp_ctx.func = func.into_owned();
+        let isa = context.isa.expect("preopt needs an ISA");
+
+        comp_ctx.flowgraph();
+        comp_ctx.preopt(isa).map_err(|e| {
+            pretty_error(&comp_ctx.func, context.isa, Into::into(e))
+        })?;
+
+        let mut text = String::new();
+        write!(&mut text, "{}", &comp_ctx.func).map_err(
+            |e| e.to_string(),
+        )?;
+        run_filecheck(&text, context)
+    }
+}