test_aggregation: expect 8 assertions with stat correction

When W2 records success for 'h', we subtract W1's delta for 'h',
so aggregated assertions are 3+6-1=8.

Author: kangze-jia

ruby/test/minitest/queue/build_status_recorder_test.rb

@@ -40,7 +40,7 @@ def test_aggregation
       reserve(second_queue, "h")
       second_reporter.record(result('h', skipped: true, requeued: true))
 
-      assert_equal 9, summary.assertions
+      assert_equal 8, summary.assertions
       assert_equal 3, summary.failures
       assert_equal 3, summary.errors
       assert_equal 1, summary.skips