test/-ext-/bug_reporter/test_bug_reporter.rb: fix race

Signal delivery is not immediate, so wait for signal delivery.

* test/-ext-/bug_reporter/test_bug_reporter.rb
  (test_bug_reporter_add): fix race

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47972 b2dd03c8-39d4-4d8f-98ff-823fe69b080e