Add test for subject

spec/rspec/expectations/subject_spec.cr

@@ -0,0 +1,12 @@
+require "../../spec_helper"
+class Base; end
+
+Spectator.describe "Subject" do
+  subject() { Base.new }
+
+  describe "#foo" do
+    it "bar" do
+      expect(subject).to be_a(Base)
+    end
+  end
+end