test/ruby: fix leaked threads

* test/thread/test_backtrace.rb: join work threads not to leak
  threads.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46132 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_backtrace.rb

@@ -195,6 +195,7 @@ class TestBacktrace < Test::Unit::TestCase
       assert_equal(n, th.backtrace_locations(0, n + 1).size)
     ensure
       q << true
+      th.join
     end
   end