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).

cranelift/Cargo.toml

@@ -23,7 +23,7 @@ docopt = "0.8.0"
 serde = "1.0.8"
 serde_derive = "1.0.8"
 num_cpus = "1.5.1"
-tempdir="0.3.5"
+tempdir = "0.3.5"
 term = "0.5"
 
 [workspace]