Move subtest::new() into the parent module.

This saves on importing of all the sub-modules that implement new test
commands, and it provides a nice overview of what 'cton-util test' can
do.

cranelift/src/tools/filetest/mod.rs

@@ -4,7 +4,10 @@
 //! available test commands.
 
 use std::path::Path;
+use cton_reader::TestCommand;
 use CommandResult;
+use cat;
+use print_cfg;
 use filetest::runner::TestRunner;
 
 pub mod subtest;
@@ -34,3 +37,17 @@ pub fn run(files: Vec<String>) -> CommandResult {
 
     runner.run()
 }
+
+/// Create a new subcommand trait object to match `parsed.command`.
+///
+/// This function knows how to create all of the possible `test <foo>` commands that can appear in
+/// a .cton test file.
+fn new_subtest(parsed: &TestCommand) -> subtest::Result<Box<subtest::SubTest>> {
+    match parsed.command {
+        "cat" => cat::subtest(parsed),
+        "print-cfg" => print_cfg::subtest(parsed),
+        "domtree" => domtree::subtest(parsed),
+        "verifier" => verifier::subtest(parsed),
+        _ => Err(format!("unknown test command '{}'", parsed.command)),
+    }
+}