[rubygems/rubygems] Make `--quiet` spec independent on the specific warning

We'll be removing the warning about no gem sources, so this spec will no
longer test that warnings are hidden by `--quiet`.

Test that in another way so that we don't lose the coverage when we
drop the specific warning about no gem server sources.

https://github.com/rubygems/rubygems/commit/cce4f86d28

spec/bundler/commands/install_spec.rb

@@ -588,13 +588,31 @@ RSpec.describe "bundle install with gem sources" do
     it "should be quiet" do
       bundle "config set force_ruby_platform true"
 
+      create_file("install_with_warning.rb", <<~RUBY)
+        require "#{lib_dir}/bundler"
+        require "#{lib_dir}/bundler/cli"
+        require "#{lib_dir}/bundler/cli/install"
+
+        module RunWithWarning
+          def run
+            super
+          rescue
+            Bundler.ui.warn "BOOOOO"
+            raise
+          end
+        end
+
+        Bundler::CLI::Install.prepend(RunWithWarning)
+      RUBY
+
       gemfile <<-G
-        gem 'rack'
+        source "#{file_uri_for(gem_repo1)}"
+        gem 'non-existing-gem'
       G
 
-      bundle :install, :quiet => true, :raise_on_error => false
-      expect(err).to include("Could not find gem 'rack'")
-      expect(err).to_not include("Your Gemfile has no gem server sources")
+      bundle :install, :quiet => true, :raise_on_error => false, :env => { "RUBYOPT" => "-r#{bundled_app("install_with_warning.rb")}" }
+      expect(err).to include("Could not find gem 'non-existing-gem'")
+      expect(err).not_to include("BOOOOO")
     end
   end