Ignore default_path test cases for Ruby with custom configuration

RubyGems provides means to override their defaults, such as
`Gem.default_dir` 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_dir` could be stubbed, but testing stub
should not be the point.

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

test/rubygems/test_gem.rb

@@ -574,6 +574,8 @@ def test_self_default_exec_format_jruby
   end
 
   def test_default_path
+    original_gem_default_dir?
+
     vendordir(File.join(@tempdir, "vendor")) do
       FileUtils.rm_rf Gem.user_home
 
@@ -584,6 +586,8 @@ def test_default_path
   end
 
   def test_default_path_missing_vendor
+    original_gem_default_dir?
+
     vendordir(nil) do
       FileUtils.rm_rf Gem.user_home
 
@@ -594,6 +598,8 @@ def test_default_path_missing_vendor
   end
 
   def test_default_path_user_home
+    original_gem_default_dir?
+
     vendordir(File.join(@tempdir, "vendor")) do
       expected = [Gem.user_dir, Gem.default_dir]
 
@@ -602,6 +608,8 @@ def test_default_path_user_home
   end
 
   def test_default_path_vendor_dir
+    original_gem_default_dir?
+
     vendordir(File.join(@tempdir, "vendor")) do
       FileUtils.mkdir_p Gem.vendor_dir
 
@@ -1796,4 +1804,10 @@ def util_remove_interrupt_command
   def util_cache_dir
     File.join Gem.dir, "cache"
   end
+
+  def original_gem_default_dir?
+    unless Gem.method(:default_dir).source_location.first.match? "lib/rubygems/defaults.rb"
+      pend "Gem.default_dir differs from original RubyGems implementation"
+    end
+  end
 end