test: Introduce GITALY_TEST_BUNDLE_PATH

During test, and test setup, BUNDLE_PATH is set by GitalySetup. It
should do The Right Thing(TM), but in case it doesn't you can set
GITALY_TEST_BUNDLE_PATH to override the value.

spec/support/helpers/gitaly_setup.rb

@@ -74,6 +74,9 @@ module GitalySetup
   end
 
   def bundle_path
+    # Allow the user to override BUNDLE_PATH if they need to
+    return ENV['GITALY_TEST_BUNDLE_PATH'] if ENV['GITALY_TEST_BUNDLE_PATH']
+
     if ENV['CI']
       expand_path('vendor/gitaly-ruby')
     else