Add initial tests for dirname.

Makefile

@@ -161,6 +161,7 @@ TEST_PROGS  := \
   cat \
   cp \
   env \
+  dirname \
   factor \
   fold \
   mkdir \

test/dirname.rs

@@ -0,0 +1,28 @@
+use std::process::Command;
+use std::str;
+
+static PROGNAME: &'static str = "./dirname";
+
+#[test]
+fn test_path_with_trailing_slashes() {
+    let dir = "/root/alpha/beta/gamma/delta/epsilon/omega//";
+    let po = Command::new(PROGNAME)
+        .arg(dir)
+        .output()
+        .unwrap_or_else(|err| panic!("{}", err));
+
+    let out = str::from_utf8(&po.stdout[..]).unwrap().trim_right();
+    assert_eq!(out, "/root/alpha/beta/gamma/delta/epsilon");
+}
+
+#[test]
+fn test_path_without_trailing_slashes() {
+    let dir = "/root/alpha/beta/gamma/delta/epsilon/omega";
+    let po = Command::new(PROGNAME)
+        .arg(dir)
+        .output()
+        .unwrap_or_else(|err| panic!("{}", err));
+
+    let out = str::from_utf8(&po.stdout[..]).unwrap().trim_right();
+    assert_eq!(out, "/root/alpha/beta/gamma/delta/epsilon");
+}