[rubygems/rubygems] Allow smoother breaking changes in test env

https://github.com/rubygems/rubygems/commit/0fc3feae19

spec/bundler/support/path.rb

@@ -102,7 +102,18 @@ module Spec
     end
 
     def tmp(*path)
-      source_root.join("tmp", scope, *path)
+      tmp_root(scope).join(*path)
+    end
+
+    def tmp_root(scope)
+      source_root.join("tmp", "#{test_env_version}.#{scope}")
+    end
+
+    # Bump this version whenever you make a breaking change to the spec setup
+    # that requires regenerating tmp/.
+
+    def test_env_version
+      1
     end
 
     def scope

spec/bundler/support/rubygems_ext.rb

@@ -57,8 +57,8 @@ module Spec
       install_test_deps
 
       (2..Parallel.processor_count).each do |n|
-        source = Path.source_root.join("tmp", "1")
-        destination = Path.source_root.join("tmp", n.to_s)
+        source = Path.tmp_root("1")
+        destination = Path.tmp_root(n.to_s)
 
         FileUtils.rm_rf destination
         FileUtils.cp_r source, destination