Add a bin/rspec helper that can run specs inside components

For when a change affects multiple components or for tools relying on
bin/rspec this helper will make things a lot easier.

Will take in both paths and options and run the specs for each
component speparately keeping the options.

Example:

    $ bin/rspec core/spec/models/spree/exchange_spec.rb api/spec/features/checkout_spec.rb -f doc

Will give the following output:

```
$ cd core; bundle exec rspec -f doc /Users/elia/Code/Nebulab/solidus/core/spec/models/spree/exchange_spec.rb; cd -
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}

Randomized with seed 190

Spree::Exchange
  #description
    describes the return items' change in options
  #perform!
    creates shipments for the order with the return items exchange inventory units
    when it cannot create shipments for all items
      raises an UnableToCreateShipments error
  #display_amount
    is the total amount of all return items
  .param_key
    is expected to eq "spree_exchange"
  #to_key
    is expected to be nil

Finished in 2.22 seconds (files took 3.27 seconds to load)
6 examples, 0 failures

Randomized with seed 190

$ cd api; bundle exec rspec -f doc /Users/elia/Code/Nebulab/solidus/api/spec/features/checkout_spec.rb; cd -

Randomized with seed 55051

Api Feature Specs
  is able to checkout with the update request
  is able to checkout with individualized requests
  is able to checkout with the create request

Finished in 3.15 seconds (files took 3.3 seconds to load)
3 examples, 0 failures

Randomized with seed 55051

```

The first line is always the executed command that can be copied and
pasted as it is.

bin/build

@@ -7,38 +7,28 @@ export DB=${DB:-postgresql}
 
 bin/setup
 
-# Solidus API
 echo "***********************"
 echo "* Testing Solidus API *"
 echo "***********************"
-cd api
-bundle exec rspec spec
+bin/rspec api/spec
 
-# Solidus Backend
 echo "***************************"
 echo "* Testing Solidus Backend *"
 echo "***************************"
-cd ../backend
-bundle exec rspec spec
-bundle exec teaspoon --require=../backend/spec/teaspoon_env.rb
+bin/rspec backend/spec
+bundle exec teaspoon --require=backend/spec/teaspoon_env.rb
 
-# Solidus Core
 echo "************************"
 echo "* Testing Solidus Core *"
 echo "************************"
-cd ../core
-bundle exec rspec spec
+bin/rspec core/spec
 
-# Solidus Frontend
 echo "****************************"
 echo "* Testing Solidus Frontend *"
 echo "****************************"
-cd ../frontend
-bundle exec rspec spec
+bin/rspec frontend/spec
 
-# Solidus Sample
 echo "**************************"
 echo "* Testing Solidus Sample *"
 echo "**************************"
-cd ../sample
-bundle exec rspec spec
+bin/rspec sample/spec

bin/rspec

@@ -0,0 +1,49 @@
+#!/usr/bin/env ruby
+
+ROOT = File.expand_path('..', __dir__)
+
+LIBS = %w[
+  api
+  backend
+  core
+  frontend
+  sample
+]
+
+# Is it a spec file or an RSpec option?
+is_spec = ->(path) {
+  path = path.split(':', 2).first # ignore line info
+  File.directory?(path) ? path.include?('/spec') : path.end_with?('_spec.rb')
+}
+
+# Find the Solidus library for this path
+find_lib = ->(path) {
+  LIBS.find { |lib| path.start_with? File.join(ROOT, lib) +'/' }
+}
+
+spec_files, options = ARGV.partition(&is_spec) # Separate specs and options
+spec_files.map! { |f| File.expand_path(f) } # Let all paths be absolute
+specs = {}
+
+if spec_files.any?
+  specs = spec_files.group_by(&find_lib)
+else
+  # If no files are provided run all specs in each lib.
+  LIBS.each { |lib| specs[lib] = [] }
+end
+
+# If we weren't able to find the solidus lib for some files
+# then we have them under the nil key.
+if specs.key?(nil)
+  abort "Couldn't find a lib for these files: #{specs[nil].join(' ')}"
+end
+
+# Run specs for each lib separately
+specs.each do |lib, files|
+  Dir.chdir(lib) do
+    command = ['bundle', 'exec', 'rspec', *options, *files]
+    warn "$ cd #{lib}; #{command.join(' ')}; cd -"
+    system *command
+    exit $?.exitstatus unless $?.success?
+  end
+end