test: ensure `require "formula"` isn't run on startup

Library/Homebrew/test/global_spec.rb

+# frozen_string_literal: true
+
+describe "brew", :integration_test do
+  it "does not invoke `require \"formula\"` at startup" do
+    expect { brew "verify-formula-undefined" }
+      .to not_to_output.to_stdout
+      .and not_to_output.to_stderr
+      .and be_a_success
+  end
+end

Library/Homebrew/test/support/helper/cmd/brew-verify-formula-undefined.rb

+# frozen_string_literal: true
+
+require "cli/parser"
+
+parser = Homebrew::CLI::Parser.new do
+  usage_banner <<~EOS
+    `verify-formula-undefined`
+
+    Verifies that `require "formula"` has not been performed at startup.
+  EOS
+end
+
+parser.parse
+
+Homebrew.failed = defined?(Formula) && Formula.respond_to?(:[])

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -66,8 +66,10 @@ RSpec.shared_context "integration test" do
     env = args.last.is_a?(Hash) ? args.pop : {}
 
     # Avoid warnings when HOMEBREW_PREFIX/bin is not in PATH.
+    # Also include our extra commands directory.
     path = [
       env["PATH"],
+      (HOMEBREW_LIBRARY_PATH/"test/support/helper/cmd").realpath.to_s,
       (HOMEBREW_PREFIX/"bin").realpath.to_s,
       ENV["PATH"],
     ].compact.join(File::PATH_SEPARATOR)