Fix system test screenshots for aggregate failures

If RSpec's `:aggregate_failures` is enabled, a system spec with a failed
expectation would not automatically trigger a screenshot as expected.

This is because aggregated failures are not exposed via the typical
`RSpec.current_example.exception`. To correctly detect failures when
aggregation is enabled, we have to do more work.

This commit works around this problem by using some behind-the-scenes
knowledge of how rspec-expectations does the aggregation. Now we reach
into the thread-local storage (`RSpec::Support.failure_notifier`) and
discover whether there are in fact failures "queued up". If so, we will
consider the example to have failed and take a screenshot.

This is the same workaround as used by capybara-screenshot:
mattheworiordan/capybara-screenshot#213

Author: mattbrictson

lib/rspec/rails/example/system_example_group.rb

@@ -58,7 +58,13 @@ def after_teardown
 
         # for the SystemTesting Screenshot situation
         def passed?
-          RSpec.current_example.exception.nil?
+          return false if RSpec.current_example.exception
+          return true unless defined?(::RSpec::Expectations::FailureAggregator)
+
+          failure_notifier = ::RSpec::Support.failure_notifier
+          return true unless failure_notifier.is_a?(::RSpec::Expectations::FailureAggregator)
+
+          failure_notifier.failures.empty? && failure_notifier.other_errors.empty?
         end
 
         # @private