Fix: extension:test_app rake task should detect if Solidus engines …

…are available

Acceptance criteria
-------------------

Given a Solidus extension calls `SolidusSupport.x_available?` in its
Install generator to check if the engine is available

When the I run `bundle exec rake extension:test_app`

Then the `SolidusSupport.x_available?` should return true (since the dummy
app installed by `extension:test_app` includes the frontend, backend, and
api engines)

Bug description
---------------

When called within `bundle exec rake extension:test_app`, the install
generator of the extension is not able to detect the Solidus engines
within the dummy app, and as such, returns nil for any
`SolidusSupport.x_available?` call.

Cause
-----

The `<Extension>::Generators::InstallGenerator.start` call within the
`common:test_app` rake task is not able to detect the Solidus engines that
were just installed by the rake task to the spec/dummy directory.

Bug fix
-------

Use the `bin/rails` executable to install the extension on the dummy app.

Possibly related issues
-----------------------

solidusio/solidus_support#66

core/lib/spree/testing_support/common_rake.rb

@@ -22,9 +22,11 @@
     sh "bin/rails db:drop db:create db:migrate VERBOSE=false RAILS_ENV=test"
 
     begin
-      require "generators/#{ENV['LIB_NAME']}/install/install_generator"
+      generator_namespace = "#{ENV['LIB_NAMESPACE']&.underscore || ENV['LIB_NAME']}"
+
+      require "generators/#{generator_namespace}/install/install_generator"
       puts 'Running extension installation generator...'
-      "#{ENV['LIB_NAMESPACE'] || ENV['LIB_NAME'].camelize}::Generators::InstallGenerator".constantize.start(["--auto-run-migrations"])
+      sh "bin/rails generate #{generator_namespace}:install --auto-run-migrations"
     rescue LoadError
       # No extension generator to run
     end