Run the filetests as part of "cargo test".

Refactor the filetests harness so that it can be run as part of
`cargo test`. And begin reorganizing the test harness code in preparation
for moving it out of the src directory.
 - Test subcommand files are now named `test_*.rs`.
 - cton-util subcommand files now just export their `run` and nothing else.
 - src/filetest/mod.rs now also just exports `run` and nothing else.
 - Tests are now run in release mode (with debug assertions enabled).

lib/cretonne/src/lib.rs

@@ -19,6 +19,7 @@ pub mod entity;
 
 pub mod bforest;
 pub mod binemit;
+pub mod cfg_printer;
 pub mod cursor;
 pub mod dominator_tree;
 pub mod flowgraph;