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/tests/cton-util-test.rs

@@ -0,0 +1,34 @@
+//! Run `cton-util test` on all available testcases.
+
+use std::process::{Command, Output};
+use std::env;
+use std::path::PathBuf;
+use std::io::{self, Write};
+
+/// Returns the target directory, where we can find build artifacts
+/// and such for the current configuration.
+fn get_target_dir() -> PathBuf {
+    let mut path = env::current_exe().unwrap();
+    path.pop(); // chop off exe name
+    path.pop(); // chop off deps name
+    path
+}
+
+#[test]
+fn cton_util_test() {
+    let mut cmd = Command::new(&get_target_dir().join("cton-util"));
+    cmd.arg("test");
+
+    // We have testcases in the following directories:
+    cmd.arg("filetests");
+    cmd.arg("docs");
+
+    let Output {
+        status,
+        stdout,
+        stderr,
+    } = cmd.output().unwrap();
+    io::stdout().write(&stdout).unwrap();
+    io::stderr().write(&stderr).unwrap();
+    assert!(status.success(), "failed with exit status {}", status);
+}