Use bundler for test gem dependencies.

Closes Homebrew/homebrew#31986.
References Homebrew/homebrew#31981.

Library/Homebrew/cmd/tests.rb

@@ -2,7 +2,14 @@ module Homebrew
   def tests
     (HOMEBREW_LIBRARY/'Homebrew/test').cd do
       ENV['TESTOPTS'] = '-v' if ARGV.verbose?
-      system "rake", "deps", "test"
+      quiet_system("gem", "list", "--installed", "bundler") || \
+        system("gem", "install", "--no-ri", "--no-rdoc",
+               "--user-install", "bundler")
+      require 'rubygems'
+      ENV["PATH"] = "#{Gem.user_dir}/bin:#{ENV["PATH"]}"
+      quiet_system("bundle", "check") || \
+        system("bundle", "install", "--path", "vendor/bundle")
+      system "bundle", "exec", "rake", "test"
       exit $?.exitstatus
     end
   end

Library/Homebrew/test/.gitignore

@@ -1,2 +1,5 @@
+/.bundle/
+/bin/
 /coverage
 /prof
+/vendor/

Library/Homebrew/test/Gemfile

@@ -0,0 +1,5 @@
+source "https://rubygems.org"
+
+gem "mocha", "~> 1.1"
+gem "minitest", "~> 5.3"
+gem "rake", "~> 10.3"

Library/Homebrew/test/Gemfile.lock

@@ -0,0 +1,16 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    metaclass (0.0.4)
+    minitest (5.4.1)
+    mocha (1.1.0)
+      metaclass (~> 0.0.1)
+    rake (10.3.2)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  minitest (~> 5.3)
+  mocha (~> 1.1)
+  rake (~> 10.3)

Library/Homebrew/test/Rakefile

@@ -3,22 +3,9 @@ require 'rake/testtask'
 
 TEST_DIRECTORY = File.dirname(File.expand_path(__FILE__))
 TEST_FILES = Dir["#{TEST_DIRECTORY}/test_*.rb"]
-GEM_DEPS = {
-  "mocha" => "~> 1.1",
-  "minitest" => "~> 5.3",
-  "rake" => "~> 10.3",
-}
 
 task :default => :test
 
-task :deps do
-  GEM_DEPS.each do |dep, version|
-    `gem list --installed #{dep} -v '#{version}'`
-    next if $?.success?
-    sh "gem", "install", "--no-ri", "--no-rdoc", "--user-install", dep, "-v", version
-  end
-end
-
 Rake::TestTask.new(:test) do |t|
   t.libs << TEST_DIRECTORY
   t.test_files = TEST_FILES