test_stringio.rb: reduce retry count

* test/stringio/test_stringio.rb (test_overflow): reduce maximum
  retry count to get rid of timeout on some platforms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55544 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/stringio/test_stringio.rb

@@ -692,7 +692,7 @@ class TestStringIO < Test::Unit::TestCase
         x = "a"*0x100000
         break if [x].pack("p").unpack("i!")[0] < 0
         ary << x
-        skip if ary.size > 1000
+        skip if ary.size > 100
       end
       s = StringIO.new(x)
       s.gets("xxx", limit)