Previously, all default/bundled gems were installed into single location
such as `/usr/local/lib/ruby/gems/3.3.0+0`. However, the same location
is used for user installed gems, which is not optimal.

With this change, the default gems goes into
`/usr/local/lib/ruby/default_gems/3.3.0+0` while the bundled gems goes
into `/usr/local/lib/ruby/bundled_gems/3.3.0+0`. User installed gems
continues to land in `/usr/local/lib/ruby/gems/3.3.0+0`.

RubyGems provides means to override their defaults, such as
`Gem.default_path` method. With such modified configuration, the
following errors might be observed:

~~~
    1) Failure:
  TestGem#test_default_path_vendor_dir [/home/runner/work/ruby/ruby/src/test/rubygems/test_gem.rb:612]:
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-qs43ch/gemhome",
   "/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-qs43ch/vendor/gems/3.3.0+0"]> expected but was
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-qs43ch/gemhome",
   "/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-qs43ch/vendor/gems/3.3.0+0",
   "/usr/local/lib/ruby/bundled_gems/3.3.0+0",
   "/usr/local/lib/ruby/default_gems/3.3.0+0"]>.
  make: *** [uncommon.mk:943: yes-test-all] Error 4

    2) Failure:
  TestGem#test_default_path_missing_vendor [/home/runner/work/ruby/ruby/src/test/rubygems/test_gem.rb:592]:
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-c41yb7/gemhome"]> expected but was
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-c41yb7/gemhome",
   "/usr/local/lib/ruby/bundled_gems/3.3.0+0",
   "/usr/local/lib/ruby/default_gems/3.3.0+0"]>.

    3) Failure:
  TestGem#test_default_path_user_home [/home/runner/work/ruby/ruby/src/test/rubygems/test_gem.rb:600]:
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-5mm6uc/userhome/.local/share/gem/ruby/3.3.0+0",
   "/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-5mm6uc/gemhome"]> expected but was
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-5mm6uc/userhome/.local/share/gem/ruby/3.3.0+0",
   "/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-5mm6uc/gemhome",
   "/usr/local/lib/ruby/bundled_gems/3.3.0+0",
   "/usr/local/lib/ruby/default_gems/3.3.0+0"]>.

    4) Failure:
  TestGem#test_default_path [/home/runner/work/ruby/ruby/src/test/rubygems/test_gem.rb:582]:
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-kan9o5/gemhome"]> expected but was
  <["/home/runner/work/ruby/ruby/build/tmp/test_rubygems_20231110-59661-kan9o5/gemhome",
   "/usr/local/lib/ruby/bundled_gems/3.3.0+0",
   "/usr/local/lib/ruby/default_gems/3.3.0+0"]>.
~~~

Of course the `Gem.default_path` could be stubbed, but testing stub
should not be the point.

Therefore, just detect if `Gem.default_path` was customized and ignore
the test cases.