Fix test to verify Bundler.setup respects custom gemfile setting

The test was only checking that `bundle install` works with a custom
gemfile, but the purpose of this PR is to make `Bundler.setup` respect
the setting too. Updated the test to actually invoke Bundler.setup and
verify it loads gems from the custom gemfile.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

spec/runtime/setup_spec.rb

@@ -307,19 +307,26 @@ def clean_load_path(lp)
     context "user sets it via `config set --local gemfile`" do
       it "uses the value in the config" do
         gemfile <<-G
-          source "#{file_uri_for(gem_repo1)}"
-          gem "rack"
+          source "https://gem.repo1"
+          gem "myrack"
         G
 
-        gemfile bundled_app("gemfake"), <<-G
-          source "#{file_uri_for(gem_repo1)}"
+        gemfile bundled_app("CustomGemfile"), <<-G
+          source "https://gem.repo1"
           gem "activesupport", "2.3.5"
         G
 
-        bundle "config set --local gemfile #{bundled_app("gemfake")}"
+        bundle "config set --local gemfile #{bundled_app("CustomGemfile")}"
         bundle "install"
 
-        expect(the_bundle).to include_gems "activesupport 2.3.5"
+        ruby <<-R
+          require 'bundler'
+          Bundler.setup
+          require 'activesupport'
+          puts ACTIVESUPPORT
+        R
+
+        expect(out).to eq("2.3.5")
       end
     end
   end