Add DSL tests for be_a matcher

spec/spectator/matchers/built_in/be_a_matcher_spec.cr

@@ -56,6 +56,62 @@ Spectator.describe Spectator::Matchers::BuiltIn::BeAMatcher do
           expect(42).to be_a(Int32)
         end.to be_successful
       end
+
+      it "does not match if the value is not an instance of the type" do
+        expect do
+          expect("foo").to be_a(Int32)
+        end.not_to be_successful
+      end
+
+      it "does not match if the value is nil" do
+        expect do
+          expect(nil).to be_a(Int32)
+        end.not_to be_successful
+      end
+
+      it "matches if the value is a subclass of the type" do
+        expect do
+          expect(Child.new).to be_a(Base)
+        end.to be_successful
+      end
+
+      it "does not match if the value is a parent of the type" do
+        expect do
+          expect(Base.new).to be_a(Child)
+        end.not_to be_successful
+      end
+    end
+
+    describe "be_an" do
+      it "matches if the value is an instance of the type" do
+        expect do
+          expect(42).to be_an(Int32)
+        end.to be_successful
+      end
+
+      it "does not match if the value is not an instance of the type" do
+        expect do
+          expect("foo").to be_an(Int32)
+        end.not_to be_successful
+      end
+
+      it "does not match if the value is nil" do
+        expect do
+          expect(nil).to be_an(Int32)
+        end.not_to be_successful
+      end
+
+      it "matches if the value is a subclass of the type" do
+        expect do
+          expect(Child.new).to be_an(Base)
+        end.to be_successful
+      end
+
+      it "does not match if the value is a parent of the type" do
+        expect do
+          expect(Base.new).to be_an(Child)
+        end.not_to be_successful
+      end
     end
   end
 end