Add testing snippets

Snippet is a self-contained example that defines the configuration,
Rails project code and specs to run.

It:
 - allows for clean separation between different snippets
 - works quite fast
 - reuses the already installed gems

In theory snippets retain the ability to use generators and arbitrary
commands, but it makes the case under test less evident.

Rakefile

@@ -224,7 +224,16 @@ end
 
 task acceptance: ['smoke:app', 'no_active_record:smoke:app', :cucumber]
 
-task default: [:spec, :acceptance]
+desc "Run short, self-contained snippets"
+task :snippets do
+  sh "echo Running snippets"
+  Dir['snippets/*.rb'].each do |snippet|
+    sh "echo Running #{snippet}"
+    sh "ruby #{snippet}"
+  end
+end
+
+task default: [:spec, :acceptance, :snippets]
 
 task :verify_private_key_present do
   private_key = File.expand_path('~/.gem/rspec-gem-private_key.pem')

script/custom_build_functions.sh

@@ -18,3 +18,7 @@ function run_cukes {
 function run_all_spec_suites {
   fold "one-by-one specs" run_specs_one_by_one
 }
+
+function run_snippets {
+  bin/rake snippets
+}

script/run_build

@@ -20,6 +20,8 @@ fi
 
 fold "cukes" run_cukes
 
+fold "snippets" run_snippets
+
 if documentation_enforced; then
   fold "doc check" check_documentation_coverage
 fi

snippets/use_active_record_false.rb

@@ -0,0 +1,39 @@
+require "bundler/inline"
+
+gemfile(true) do
+  source "https://rubygems.org"
+
+  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
+
+  # Those Gemfiles carefully pick the right versions depending on
+  # settings in the ENV, `.rails-version` and `maintenance-branch`.
+  eval_gemfile 'Gemfile-rspec-dependencies'
+  eval_gemfile 'Gemfile-rails-dependencies'
+
+  gem "rspec-rails", path: "./"
+  gem "sqlite3"
+  gem "ammeter"
+end
+
+# Initialization
+require "active_record/railtie"
+require "rspec/rails"
+
+# Run specs on exit
+require "rspec/autorun"
+
+# RSpec configuration
+RSpec.configure do |config|
+  config.use_active_record = false
+end
+
+# Rails project code
+class Command
+end
+
+# Rails project specs
+RSpec.describe Command do
+  it 'does not not break' do
+    Command.new
+  end
+end