diff --git a/test/helper.rb b/test/helper.rb
index 936638d2..78b1f78b 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -42,6 +42,22 @@ Minitest::Reporters.use! [
   )
 ]
 
+module Minitest::Assertions
+  def assert_exist(filename, msg = nil)
+    msg = message(msg) {
+      "Expected '#{filename}' to exist"
+    }
+    assert File.exist?(filename), msg
+  end
+
+  def refute_exist(filename, msg = nil)
+    msg = message(msg) {
+      "Expected '#{filename}' not to exist"
+    }
+    refute File.exist?(filename), msg
+  end
+end
+
 class JekyllUnitTest < Minitest::Test
   include ::RSpec::Mocks::ExampleMethods