[rubygems/rubygems] Resolve @@project_dir from test file paths

`Dir.pwd` may differ from the source path.  Test directories and
files should be resolved from test file paths.

https://github.com/rubygems/rubygems/commit/e18e7c81b4

lib/rubygems/test_case.rb

@@ -232,8 +232,6 @@ class Gem::TestCase < (defined?(Minitest::Test) ? Minitest::Test : MiniTest::Uni
   undef_method :default_test if instance_methods.include? 'default_test' or
                                 instance_methods.include? :default_test
 
-  @@project_dir = Dir.pwd.untaint unless defined?(@@project_dir)
-
   ##
   # #setup prepares a sandboxed location to install gems.  All installs are
   # directed to a temporary directory.  All install plugins are removed.

test/rubygems/test_gem.rb

@@ -17,6 +17,8 @@ class TestGem < Gem::TestCase
 
   PLUGINS_LOADED = [] # rubocop:disable Style/MutableConstant
 
+  @@project_dir = File.expand_path('../../..', __FILE__).untaint
+
   def setup
     super
 

test/rubygems/test_gem_command_manager.rb

@@ -4,6 +4,8 @@ require 'rubygems/command_manager'
 
 class TestGemCommandManager < Gem::TestCase
 
+  @@project_dir = File.expand_path('../../..', __FILE__).untaint
+
   def setup
     super