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/docs/testing.rst

@@ -324,6 +324,22 @@ Test the simple GVN pass.
 The simple GVN pass is run on each function, and then results are run
 through filecheck.
 
+`test licm`
+-----------------
+
+Test the LICM pass.
+
+The LICM pass is run on each function, and then results are run
+through filecheck.
+
+`test preopt`
+-----------------
+
+Test the preopt pass.
+
+The preopt pass is run on each function, and then results are run
+through filecheck.
+
 `test compile`
 --------------