Add `formula` spec helper.

Library/Homebrew/test/spec_helper.rb

@@ -16,6 +16,7 @@ require "tap"
 
 require "test/support/helper/shutup"
 require "test/support/helper/fixtures"
+require "test/support/helper/formula"
 require "test/support/helper/spec/shared_context/integration_test"
 
 TEST_DIRECTORIES = [
@@ -32,6 +33,7 @@ RSpec.configure do |config|
   config.order = :random
   config.include(Test::Helper::Shutup)
   config.include(Test::Helper::Fixtures)
+  config.include(Test::Helper::Formula)
   config.before(:each) do |example|
     if example.metadata[:needs_macos]
       skip "Not on macOS." unless OS.mac?

Library/Homebrew/test/support/helper/formula.rb

+require "formulary"
+
+module Test
+  module Helper
+    module Formula
+      def formula(name = "formula_name", path: Formulary.core_path(name), spec: :stable, alias_path: nil, &block)
+        Class.new(::Formula, &block).new(name, path, spec, alias_path: alias_path)
+      end
+
+      # Use a stubbed {Formulary::FormulaLoader} to make a given formula be found
+      # when loading from {Formulary} with `ref`.
+      def stub_formula_loader(formula, ref = formula.full_name)
+        loader = double(get_formula: formula)
+        allow(Formulary).to receive(:loader_for).with(ref, from: :keg).and_return(loader)
+        allow(Formulary).to receive(:loader_for).with(ref, from: nil).and_return(loader)
+      end
+    end
+  end
+end