Make test dependencies conditional so we can enable them when running autopkgtest

--- a/Gemfile
+++ b/Gemfile
@@ -364,7 +364,7 @@
 
 gem 'warning', '~> 1.3'
 
-group :test do
+if ENV["INCLUDE_TEST_DEPENDS"] == "true"
   gem 'fuubar', '~> 2.2.0'
   gem 'rspec-retry', '~> 0.6.2'
   gem 'rspec_profiling', '~> 0.0.6'