Add specification of chain matcher .build

spec/unit/mutant/matcher/chain_spec.rb

@@ -43,4 +43,18 @@ describe Mutant::Matcher::Chain do
     it_should_behave_like 'an idempotent method'
   end
 
+  describe '.build' do
+    subject { described_class.build(matchers) }
+
+    context 'when one matcher given' do
+      let(:matchers) { [double('Matcher A')] }
+      it { should be(matchers.first) }
+    end
+
+    context 'when matchers given' do
+      let(:matchers) { [double('Matcher A'), double('Matcher B')] }
+      it { should eql(described_class.new(matchers)) }
+    end
+  end
+
 end