Keep Homebrew site-packages in sys.path during brew test

Formulas that build python things and which are tested with system
Python will encounter test failures unless system Python is configured
to add Homebrew's site-packages to sys.path. This change makes sure that
configuration is performed in the test environment.

Both lines are needed; the first reads and processes .pth files and the
second makes sure that Homebrew's site-packages is read before the
system extras, so that formulas depending on Homebrew/python/numpy get
the Homebrew/python version and not the old system version.

Closes Homebrew/homebrew#38466.

Library/Homebrew/formula.rb

@@ -634,6 +634,7 @@ class Formula
     mktemp do
       @testpath = Pathname.pwd
       ENV["HOME"] = @testpath
+      setup_test_home @testpath
       test
     end
   ensure
@@ -658,6 +659,16 @@ class Formula
 
   protected
 
+  def setup_test_home home
+    # keep Homebrew's site-packages in sys.path when testing with system Python
+    user_site_packages = home/"Library/Python/2.7/lib/python/site-packages"
+    user_site_packages.mkpath
+    (user_site_packages/"homebrew.pth").write <<-EOS.undent
+      import site; site.addsitedir("#{HOMEBREW_PREFIX}/lib/python2.7/site-packages")
+      import sys; sys.path.insert(0, "#{HOMEBREW_PREFIX}/lib/python2.7/site-packages")
+    EOS
+  end
+
   # Pretty titles the command and buffers stdout/stderr
   # Throws if there's an error
   def system cmd, *args