Add initial tests for default values

Makefile

@@ -143,6 +143,7 @@ TEST_PROGS  := \
 	stdbuf \
 	sum \
 	tac \
+	tail \
 	test \
 	touch \
 	tr \

tests/fixtures/tail/foobar.txt

@@ -0,0 +1,11 @@
+baz
+foo
+bar
+foo
+bar
+foo
+bar
+foo
+bar
+foo
+bar

tests/fixtures/tail/foobar_single_default.expected

@@ -0,0 +1,10 @@
+foo
+bar
+foo
+bar
+foo
+bar
+foo
+bar
+foo
+bar

tests/fixtures/tail/foobar_stdin_default.expected

@@ -0,0 +1,10 @@
+foo
+bar
+foo
+bar
+foo
+bar
+foo
+bar
+foo
+bar

tests/tail.rs

@@ -0,0 +1,23 @@
+#[macro_use]
+mod common;
+
+use common::util::*;
+
+static UTIL_NAME: &'static str = "tail";
+
+static INPUT: &'static str = "foobar.txt";
+
+
+#[test]
+fn test_stdin_default() {
+    let (at, mut ucmd) = testing(UTIL_NAME);
+    let result = ucmd.run_piped_stdin(at.read(INPUT));
+    assert_eq!(result.stdout, at.read("foobar_stdin_default.expected"));
+}
+
+#[test]
+fn test_single_default() {
+    let (at, mut ucmd) = testing(UTIL_NAME);
+    let result = ucmd.arg(INPUT).run();
+    assert_eq!(result.stdout, at.read("foobar_single_default.expected"));
+}