[rubygems/rubygems] Add some early assertions to make sure the test is progressing fine

If an error happens during the install command, it will fail in an
strange way right now.

https://github.com/rubygems/rubygems/commit/2b6e0c703a

test/rubygems/test_gem_commands_pristine_command.rb

@@ -392,6 +392,9 @@ class TestGemCommandsPristineCommand < Gem::TestCase
     b = util_spec "b"
     install_gem b
 
+    assert_path_exist File.join(gemhome2, "gems", "b-2")
+    assert_path_not_exist File.join(@gemhome, "gems", "b-2")
+
     @cmd.options[:args] = %w[a b]
 
     use_ui @ui do