Add unit test for custom Liquid tag registration (#1960)

Adds EnvironmentTest to verify custom tag registration and rendering.

test/unit/environment_test.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class EnvironmentTest < Minitest::Test
+  include Liquid
+
+  class UnsubscribeFooter < Liquid::Tag
+    def render(_context)
+      'Unsubscribe Footer'
+    end
+  end
+
+  def test_custom_tag
+    email_environment = Liquid::Environment.build do |environment|
+      environment.register_tag("unsubscribe_footer", UnsubscribeFooter)
+    end
+
+    assert(email_environment.tags["unsubscribe_footer"])
+    assert(email_environment.tag_for_name("unsubscribe_footer"))
+    template = Liquid::Template.parse("{% unsubscribe_footer %}", environment: email_environment)
+
+    assert_equal('Unsubscribe Footer', template.render)
+  end
+end