Fix brew tests on High Sierra.

The `brew test` tests were failing as they were unable to include
`test/unit/assertions`. This is because it's a gem and we were setting
the `GEM_HOME` so system gems were being ignored.

While I was there and examining the `$LOAD_PATH`: reduce the number of
things we add there by using `Bundler.with_clean_env` and only adding
`HOMEBREW_LIBRARY_PATH` in the `$LOAD_PATH` if it isn't already there
(which it always is [and has to be] for integration tests).

This also seems to have the side effect of speeding up integration
tests from 1m26s to 1m8s on my machine.

Library/Homebrew/brew.rb

@@ -11,7 +11,9 @@ raise "Homebrew must be run under Ruby 2!" unless RUBY_TWO
 require "pathname"
 HOMEBREW_LIBRARY_PATH = Pathname.new(__FILE__).realpath.parent
 require "English"
-$LOAD_PATH.unshift(HOMEBREW_LIBRARY_PATH.to_s)
+unless $LOAD_PATH.include?(HOMEBREW_LIBRARY_PATH.to_s)
+  $LOAD_PATH.unshift(HOMEBREW_LIBRARY_PATH.to_s)
+end
 require "global"
 require "tap"
 

Library/Homebrew/test/support/helper/spec/shared_context/integration_test.rb

@@ -77,6 +77,7 @@ RSpec.shared_context "integration test" do
       "HOMEBREW_INTEGRATION_TEST" => command_id_from_args(args),
       "HOMEBREW_TEST_TMPDIR" => TEST_TMPDIR,
       "HOMEBREW_DEVELOPER" => ENV["HOMEBREW_DEVELOPER"],
+      "GEM_HOME" => nil,
     )
 
     ruby_args = [
@@ -89,7 +90,7 @@ RSpec.shared_context "integration test" do
     ruby_args << "-rtest/support/helper/integration_mocks"
     ruby_args << (HOMEBREW_LIBRARY_PATH/"brew.rb").resolved_path.to_s
 
-    Bundler.with_original_env do
+    Bundler.with_clean_env do
       stdout, stderr, status = Open3.capture3(env, RUBY_PATH, *ruby_args, *args)
       $stdout.print stdout
       $stderr.print stderr