HOMEBREW_BREW_FILE was unset for the tests

This fixes a regression introduced by 3f6a355, which caused
HOMEBREW_BREW_FILE to be unset while running the tests.

Patch provided by Xu Cheng.

Library/Homebrew/test/lib/config.rb

@@ -1,6 +1,7 @@
 require "tmpdir"
 require "pathname"
 
+HOMEBREW_BREW_FILE = ENV["HOMEBREW_BREW_FILE"]
 HOMEBREW_TEMP = Pathname.new(ENV["HOMEBREW_TEMP"] || Dir.tmpdir)
 
 TEST_TMPDIR = ENV.fetch("HOMEBREW_TEST_TMPDIR") { |k|

Library/Homebrew/test/test_integration_cmds.rb

@@ -7,6 +7,12 @@ require "pathname"
 class IntegrationCommandTests < Homebrew::TestCase
   def setup
     @cmd_id_index = 0 # Assign unique IDs to invocations of `cmd_output`.
+    (HOMEBREW_PREFIX/"bin").mkpath
+    FileUtils.touch HOMEBREW_PREFIX/"bin/brew"
+  end
+
+  def teardown
+    (HOMEBREW_PREFIX/"bin").rmtree
   end
 
   def cmd_id_from_args(args)