Fix ruby-head failing tests

In `ruby-head` (where system rubygems already has the `XDG` standard
implementation), some tests currently depend on the presence of a
`~/.gem` folder in the home of the user that runs the tests. If that
file is present, tests pass, otherwise they don't.

For example, the following passes if you have a `~/.gem` folder but
fails otherwise with:

```
$ rake TESTOPTS="--name=/TestGemCommandsGenerateIndexCommand#test_execute$\|TestGemCommandsUpdateCommand#test_execute_user_install/ -v"
Run options: "--name=/TestGemCommandsGenerateIndexCommand#test_execute$|TestGemCommandsUpdateCommand#test_execute_user_install/" -v --seed 17318

# Running:

TestGemCommandsGenerateIndexCommand#test_execute = 0.02 s = .
TestGemCommandsUpdateCommand#test_execute_user_install = /rubygems/test/rubygems/test_gem_commands_update_command.rb:412: warning: instance variable @user_install not initialized
0.04 s = F

Finished in 0.095337s, 20.9783 runs/s, 20.9783 assertions/s.

  1) Failure:
TestGemCommandsUpdateCommand#test_execute_user_install [/rubygems/test/rubygems/test_gem_commands_update_command.rb:414]:
user_install must be set on the installer

2 runs, 2 assertions, 1 failures, 0 errors, 0 skips
rake aborted!
Command failed with status (1)

Tasks: TOP => default => test
(See full trace by running task with --trace)

```

This is because the very initial `require` of the default `did_you_mean`
gem that ruby does on startup runs _before_  the global `setup` hook of
our tests run. During this require `Gem.data_home` and its value is
memoized to a path in the real users home (not the fake user's home that
our tests setup, since that code hasn't run yet). Then that memoized
value is used when looking for the default folders to look for gems, and
since there's no `~/.gem` folder, its value is actually used as part of
the `Gem.user_dir` folder in `Gem::Specification.dirs` (this is how
we've approached backwards compatibility for the `XDG` feature). That
means dummy test gems with the `--user-install` flag are installed to
global, real locations and everything is messed up.

This commit fixes the issue by resetting the `Gem.data_home` value in
case it has already been memoized.

lib/rubygems/test_case.rb

@@ -357,6 +357,7 @@ class Gem::TestCase < Minitest::Test
 
     ENV['HOME'] = @userhome
     Gem.instance_variable_set :@user_home, nil
+    Gem.instance_variable_set :@data_home, nil
     Gem.instance_variable_set :@gemdeps, nil
     Gem.instance_variable_set :@env_requirements_by_name, nil
     Gem.send :remove_instance_variable, :@ruby_version if