Implement rspec TestEnvHelpers:

Allow use of our test environment, with a mocked remote and convenient
methods to write more concise expectations.

  #env, builds and returns a memoized test environment (with mocked
  remote)

  #output, returns env output as a string

  #remote_fs, returns env remote fs

  #expect_execution, setup a remote command execution expectation

spec/support/shared_test.rb

@@ -1,6 +1,7 @@
 module Producer::Core
   shared_examples 'test' do
-    let(:env)         { Env.new }
+    include TestEnvHelpers
+
     let(:arguments)   { [:some, :arguments] }
     subject(:test)    { described_class.new(env, *arguments) }