Add a SubTest trait and filecheck utilities.

This trait serves as a shared interface for the different kinds of test
commands the 'cton-util test' understands.

Many tests produce output that is run through filecheck for validation.
Provide a simple run_filecheck() function to help with this.

Implement the 'test cat' sub-test which is probably the simplest
possible.

cranelift/src/tools/filetest/mod.rs

@@ -7,6 +7,7 @@ use std::path::Path;
 use CommandResult;
 use filetest::runner::TestRunner;
 
+pub mod subtest;
 mod runner;
 
 /// Main entry point for `cton-util test`.