Add specs for Subject#tests

spec/unit/mutant/subject_spec.rb

@@ -2,7 +2,11 @@ RSpec.describe Mutant::Subject do
   let(:class_under_test) do
     Class.new(described_class) do
       def expression
-        Mutant::Expression.parse('Test')
+        Mutant::Expression.parse('SubjectA')
+      end
+
+      def match_expressions
+        [expression] << Mutant::Expression.parse('SubjectB')
       end
     end
   end
@@ -34,7 +38,7 @@ RSpec.describe Mutant::Subject do
   describe '#identification' do
     subject { object.identification }
 
-    it { should eql('Test:source_path:source_line') }
+    it { should eql('SubjectA:source_path:source_line') }
   end
 
   describe '#prepare' do
@@ -43,6 +47,56 @@ RSpec.describe Mutant::Subject do
     it_should_behave_like 'a command method'
   end
 
+  describe '#tests' do
+    let(:config)      { Mutant::Config::DEFAULT.update(integration: integration)         }
+    let(:integration) { double('Integration', all_tests: all_tests)                      }
+    let(:test_a)      { double('test', expression: Mutant::Expression.parse('SubjectA')) }
+    let(:test_b)      { double('test', expression: Mutant::Expression.parse('SubjectB')) }
+    let(:test_c)      { double('test', expression: Mutant::Expression.parse('SubjectC')) }
+
+    subject { object.tests }
+
+    context 'without available tests' do
+      let(:all_tests) { [] }
+
+      it { should eql([]) }
+
+      it_should_behave_like 'an idempotent method'
+    end
+
+    context 'without qualifying tests' do
+      let(:all_tests) { [test_c] }
+
+      it { should eql([]) }
+
+      it_should_behave_like 'an idempotent method'
+    end
+
+    context 'with qualifying tests for first match expression' do
+      let(:all_tests) { [test_a] }
+
+      it { should eql([test_a]) }
+
+      it_should_behave_like 'an idempotent method'
+    end
+
+    context 'with qualifying tests for second match expression' do
+      let(:all_tests) { [test_b] }
+
+      it { should eql([test_b]) }
+
+      it_should_behave_like 'an idempotent method'
+    end
+
+    context 'with qualifying tests for the first and second match expression' do
+      let(:all_tests) { [test_a, test_b] }
+
+      it { should eql([test_a]) }
+
+      it_should_behave_like 'an idempotent method'
+    end
+  end
+
   describe '#node' do
     subject { object.node }