tail: test following multiple files

The test_follow_multiple() test verifies that input is read upon append
on both files and that headers are printed when expected.

tests/fixtures/tail/foobar2.txt

@@ -0,0 +1,2 @@
+un
+deux

tests/fixtures/tail/foobar_follow_multiple.expected

@@ -0,0 +1,15 @@
+==> foobar.txt <==
+dos
+tres
+quattro
+cinco
+seis
+siette
+ocho
+nueve
+diez
+once
+
+==> foobar2.txt <==
+un
+deux

tests/fixtures/tail/foobar_follow_multiple_appended.expected

@@ -0,0 +1,4 @@
+
+==> foobar.txt <==
+doce
+trece

tests/test_tail.rs

@@ -8,6 +8,7 @@ use uu_tail::parse_size;
 static UTIL_NAME: &'static str = "tail";
 
 static FOOBAR_TXT: &'static str = "foobar.txt";
+static FOOBAR_2_TXT: &'static str = "foobar2.txt";
 static FOOBAR_WITH_NULL_TXT: &'static str = "foobar_with_null.txt";
 
 #[test]
@@ -56,6 +57,26 @@ fn test_follow() {
     child.kill().unwrap();
 }
 
+#[test]
+fn test_follow_multiple() {
+    let (at, mut ucmd) = testing(UTIL_NAME);
+    let mut child = ucmd.arg("-f").arg(FOOBAR_TXT).arg(FOOBAR_2_TXT).run_no_wait();
+
+    let expected = at.read("foobar_follow_multiple.expected");
+    assert_eq!(read_size(&mut child, expected.len()), expected);
+
+    let first_append = "trois\n";
+    at.append(FOOBAR_2_TXT, first_append);
+    assert_eq!(read_size(&mut child, first_append.len()), first_append);
+
+    let second_append = "doce\ntrece\n";
+    let expected = at.read("foobar_follow_multiple_appended.expected");
+    at.append(FOOBAR_TXT, second_append);
+    assert_eq!(read_size(&mut child, expected.len()), expected);
+
+    child.kill().unwrap();
+}
+
 #[test]
 fn test_single_big_args() {
     const FILE: &'static str = "single_big_args.txt";