Add initial test for pwd.

Makefile

@@ -170,6 +170,7 @@ TEST_PROGS  := \
   mv \
   nl \
   paste \
+  pwd \
   seq \
   sort \
   test \

test/pwd.rs

@@ -0,0 +1,16 @@
+use std::env;
+use std::process::Command;
+use std::str;
+
+static PROGNAME: &'static str = "./pwd";
+
+#[test]
+fn test_default() {
+    let po = Command::new(PROGNAME)
+        .output()
+        .unwrap_or_else(|err| panic!("{}", err));
+
+    let out = str::from_utf8(&po.stdout[..]).unwrap().trim_right();
+    let expected = env::current_dir().unwrap().into_os_string().into_string().unwrap();
+    assert_eq!(out, expected);
+}