xtail: fix test (#5481)

Formula/xtail.rb

@@ -22,6 +22,26 @@ class Xtail < Formula
   end
 
   test do
-    system "#{bin}/xtail"
+    file1 = testpath/"file1"
+    file2 = testpath/"file2"
+    touch file1
+    touch file2
+
+    begin
+      p = IO.popen("#{bin}/xtail file1 file2")
+      # Give xtail a couple seconds before and after so that it could
+      # relatively reliably pick up the changes.
+      sleep 2
+      file1.append_lines "hello\n"
+      file2.append_lines "world\n"
+      sleep 2
+    ensure
+      Process.kill "QUIT", p.pid
+      Process.wait p.pid
+    end
+
+    output = p.read
+    assert_match "hello", output
+    assert_match "world", output
   end
 end