test_fork.rb: wait signals

* bootstraptest/test_fork.rb: wait until receiving both singals,
  upto 0.1sec.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46606 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-06-29

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46607 b2dd03c8-39d4-4d8f-98ff-823fe69b080e