Allow test tmpdir to be passed in

Library/Homebrew/test/lib/config.rb

@@ -1,3 +1,11 @@
+require "tmpdir"
+
+TEST_TMPDIR = ENV.fetch("HOMEBREW_TEST_TMPDIR") { |k|
+  dir = Dir.mktmpdir("homebrew_tests")
+  at_exit { FileUtils.remove_entry(dir) }
+  ENV[k] = dir
+}
+
 HOMEBREW_PREFIX        = Pathname.new(TEST_TMPDIR).join("prefix")
 HOMEBREW_REPOSITORY    = HOMEBREW_PREFIX
 HOMEBREW_LIBRARY       = HOMEBREW_REPOSITORY+"Library"

Library/Homebrew/test/testing_env.rb

@@ -1,11 +1,6 @@
 $:.unshift File.expand_path("../..", __FILE__)
 $:.unshift File.expand_path("../lib", __FILE__)
 
-require "tmpdir"
-
-TEST_TMPDIR = Dir.mktmpdir("homebrew_tests")
-at_exit { FileUtils.remove_entry(TEST_TMPDIR) }
-
 require "global"
 
 # Test environment setup