test: add tests from previous dev steps

Used these tests in developing things but forgot to add them.

test/policies/post_policy_test.rb

+require 'test_helper'
+
+class PostPolicyTest < ActiveSupport::TestCase
+
+  def test_admin_may_delete_visitor_comment
+    admin = users(:blue)
+    visitor = users(:penguin)
+    page = FactoryBot.create :page, created_by: admin,
+      owner: groups(:rainbow),
+      public: true
+    visitor_comment = page.add_post visitor,
+      body: 'test comment by penguin on public page'
+    policy = Pundit.policy!(admin, visitor_comment)
+    assert policy.destroy?
+  end
+
+end

test/unit/asset/corrupt_test.rb

+require 'test_helper'
+
+class Asset::PdfTest < ActiveSupport::TestCase
+  def setup
+    setup_assets
+  end
+
+  def teardown
+    teardown_assets
+  end
+
+  def test_corrupt_file_upload
+    @asset = Asset.create_from_params uploaded_data: upload_data('corrupt.jpg')
+    @asset.generate_thumbnails
+    @asset.thumbnails.each do |thumb|
+      refute thumb.ok?, format('thumbnail "%s" should have failed for corrupt file', thumb.name)
+      assert thumb.private_filename, format('thumbnail "%s" should exist', thumb.name)
+    end
+  end
+end