Add feature spec when generating generator

The before hook help us to redo the test and clean the files

features/generator_specs/generator_specs.feature

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+Feature: Generator spec
+
+    RSpec spec are normally generated alongside other application components.
+    For instance, `rails generate model` will also generate an RSpec spec file
+    for the model but you can also use your own generator. See
+    [customizing your workflow](https://guides.rubyonrails.org/generators.html#customizing-your-workflow)
+
+    @remove-generator-files
+    Scenario: Use custom generator without '--generator-specs' arg
+        When I run `bundle exec rails generate generator my_generator`
+        Then the features should pass
+        Then the output should contain:
+          """
+                create  lib/generators/my_generator
+                create  lib/generators/my_generator/my_generator_generator.rb
+                create  lib/generators/my_generator/USAGE
+                create  lib/generators/my_generator/templates
+                invoke  rspec
+          """
+
+    @remove-generator-files
+    Scenario: Use custom generator with '--generator-specs' arg
+        When I run `bundle exec rails generate generator my_generator --generator-specs`
+        Then the features should pass
+        Then the output should contain:
+          """
+                create  lib/generators/my_generator
+                create  lib/generators/my_generator/my_generator_generator.rb
+                create  lib/generators/my_generator/USAGE
+                create  lib/generators/my_generator/templates
+                invoke  rspec
+                create    spec/generator/my_generators_generator_spec.rb
+          """

features/support/hooks.rb

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+Before('@remove-generator-files') do
+  command = <<-COMMAND
+  rm -f \
+    tmp/example_app/lib/generators/my_generator \
+    tmp/example_app/lib/generators/my_generator/my_generator_generator.rb \
+    tmp/example_app/lib/generators/my_generator/USAGE \
+    tmp/example_app/lib/generators/my_generator/templates \
+    tmp/example_app/spec/generator/my_generator_spec.rb
+  COMMAND
+  system(command)
+end