Add spec for wiki instantiation

spec/models/wiki_spec.rb

+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe Wiki do
+  describe '.new' do
+    it 'verifies that the user is a User' do
+      expect { described_class.new(double, 1) }.to raise_error(ArgumentError)
+      expect { described_class.new(double, build(:group)) }.to raise_error(ArgumentError)
+      expect { described_class.new(double, build(:user)) }.not_to raise_error
+      expect { described_class.new(double, nil) }.not_to raise_error
+    end
+  end
+end