Fix specification holes in Module::Cache

spec/unit/mutant/cache_spec.rb

@@ -0,0 +1,22 @@
+RSpec.describe Mutant::Cache do
+  let(:object) { described_class.new }
+
+  describe '#parse' do
+    let(:path) { double('Path') }
+
+    subject { object.parse(path) }
+
+    before do
+      allow(File).to receive(:read).with(path).and_return(':source')
+    end
+
+    it 'returns parsed source' do
+      expect(subject).to eql(s(:sym, :source))
+    end
+
+    it 'returns cached parsed source' do
+      source = object.parse(path)
+      expect(subject).to be(source)
+    end
+  end
+end