Stop to create directories in default gem directory for test

It breaks test on system installed RubyGems and normal user
environment with the following error:

      1) Error:
    test_generate_default_gem(TestRDocRubygemsHook):
    Errno::EACCES: Permission denied - /var/lib/gems/2.0.0/doc/a-2
        /usr/lib/ruby/2.0.0/fileutils.rb:245:in `mkdir'
        /usr/lib/ruby/2.0.0/fileutils.rb:245:in `fu_mkdir'
        /usr/lib/ruby/2.0.0/fileutils.rb:219:in `block (2 levels) in mkdir_p'
        /usr/lib/ruby/2.0.0/fileutils.rb:217:in `reverse_each'
        /usr/lib/ruby/2.0.0/fileutils.rb:217:in `block in mkdir_p'
        /usr/lib/ruby/2.0.0/fileutils.rb:203:in `each'
        /usr/lib/ruby/2.0.0/fileutils.rb:203:in `mkdir_p'
        /home/kou/work/ruby/rdoc/test/test_rdoc_rubygems_hook.rb:141:in `test_generate_default_gem'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:1301:in `run'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:919:in `block in _run_suite'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:912:in `map'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:912:in `_run_suite'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:899:in `block in _run_suites'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:899:in `map'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:899:in `_run_suites'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:867:in `_run_anything'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:1060:in `run_tests'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:1047:in `block in _run'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:1046:in `each'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:1046:in `_run'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:1035:in `run'
        /usr/lib/ruby/2.0.0/minitest/unit.rb:789:in `block in autorun'

We don't need to create directories because the test checks that
RubyGems hook isn't ran.