Don't use libtest harness for filetests (#4655)

We are using our own test harness for filetests and embedding it in
libtest isn't useful. It only hides test output until the end and
results in unnecessary noise.

cranelift/tests/filetests.rs

@@ -1,6 +1,5 @@
-#[test]
-fn filetests() {
+fn main() -> anyhow::Result<()> {
     // Run all the filetests in the following directories.
-    cranelift_filetests::run(false, false, &["filetests".into(), "docs".into()])
-        .expect("test harness");
+    cranelift_filetests::run(false, false, &["filetests".into(), "docs".into()])?;
+    Ok(())
 }