Implement `has_executable' condition keyword

spec/producer/core/condition/dsl_spec.rb

@@ -7,7 +7,13 @@ module Producer::Core
       let(:env)     { double 'env' }
       subject(:dsl) { DSL.new(env, &block) }
 
-      %w[file_contains has_dir has_env has_file].each do |test|
+      %w[
+        file_contains
+        has_dir
+        has_env
+        has_executable
+        has_file
+      ].each do |test|
         it "has `#{test}' test defined" do
           expect(dsl).to respond_to test.to_sym
         end

spec/producer/core/tests/has_executable_spec.rb

@@ -0,0 +1,29 @@
+require 'spec_helper'
+
+module Producer::Core
+  module Tests
+    describe HasExecutable, :env do
+      subject(:has_env) { HasExecutable.new(env, executable) }
+
+      it_behaves_like 'test'
+
+      describe '#verify' do
+        context 'executable exists' do
+          let(:executable) { 'true' }
+
+          it 'returns true' do
+            expect(has_env.verify).to be true
+          end
+        end
+
+        context 'executable does not exist' do
+          let(:executable) { 'some_non_existent_executable' }
+
+          it 'returns false' do
+            expect(has_env.verify).to be false
+          end
+        end
+      end
+    end
+  end
+end