[rubygems/rubygems] Unify specs testing Bundler.setup failing behavior

They had slightly different behavior in Bundler 2 and Bundler 3 because
Bundler 3 will auto-clean after `bundle install`, so one of them system
installed gems won't be present when `Bundler.setup` fails.

We can unify the specs by moving installing system gems to _after_
`bundle install`, so the behavior in both cases is the same. Also, only
this spec actually needs these gems installed, so we can move that from
the `before` block to the body of the spec.

https://github.com/rubygems/rubygems/commit/b7251ec9c6

spec/bundler/commands/exec_spec.rb

@@ -723,8 +723,6 @@ RSpec.describe "bundle exec" do
     RUBY
 
     before do
-      system_gems(%w[myrack-1.0.0 myrack-0.9.1], path: default_bundle_path)
-
       bundled_app(path).open("w") {|f| f << executable }
       bundled_app(path).chmod(0o755)
 
@@ -870,8 +868,10 @@ RSpec.describe "bundle exec" do
       end
     end
 
-    context "when Bundler.setup fails", bundler: "< 3" do
+    context "when Bundler.setup fails" do
       before do
+        system_gems(%w[myrack-1.0.0 myrack-0.9.1], path: default_bundle_path)
+
         gemfile <<-G
           source "https://gem.repo1"
           gem 'myrack', '2'
@@ -900,35 +900,6 @@ Run `bundle install` to install missing gems.
       end
     end
 
-    context "when Bundler.setup fails", bundler: "3" do
-      before do
-        gemfile <<-G
-          source "https://gem.repo1"
-          gem 'myrack', '2'
-        G
-        ENV["BUNDLER_FORCE_TTY"] = "true"
-      end
-
-      let(:exit_code) { Bundler::GemNotFound.new.status_code }
-      let(:expected) { "" }
-      let(:expected_err) { <<-EOS.strip }
-Could not find gem 'myrack (= 2)' in locally installed gems.
-
-The source contains the following gems matching 'myrack':
-  * myrack-1.0.0
-Run `bundle install` to install missing gems.
-      EOS
-
-      it "runs" do
-        skip "https://github.com/rubygems/rubygems/issues/3351" if Gem.win_platform?
-
-        subject
-        expect(exitstatus).to eq(exit_code)
-        expect(err).to eq(expected_err)
-        expect(out).to eq(expected)
-      end
-    end
-
     context "when Bundler.setup fails and Gemfile is not the default" do
       before do
         gemfile "CustomGemfile", <<-G