[rubygems/rubygems] Run definition specs in an isolated location

And consistently pass Pathname's to `Definition.new` like production
code does.

https://github.com/rubygems/rubygems/commit/660def5b68

spec/bundler/bundler/definition_spec.rb

@@ -5,17 +5,16 @@ require "bundler/definition"
 RSpec.describe Bundler::Definition do
   describe "#lock" do
     before do
-      allow(Bundler).to receive(:settings) { Bundler::Settings.new(".") }
-      allow(Bundler::SharedHelpers).to receive(:find_gemfile) { Pathname.new("Gemfile") }
+      allow(Bundler::SharedHelpers).to receive(:find_gemfile) { bundled_app_gemfile }
       allow(Bundler).to receive(:ui) { double("UI", info: "", debug: "") }
     end
 
-    subject { Bundler::Definition.new("Gemfile.lock", [], Bundler::SourceList.new, {}) }
+    subject { Bundler::Definition.new(bundled_app_lock, [], Bundler::SourceList.new, {}) }
 
     context "when it's not possible to write to the file" do
       it "raises an PermissionError with explanation" do
         allow(File).to receive(:open).and_call_original
-        expect(File).to receive(:open).with("Gemfile.lock", "wb").
+        expect(File).to receive(:open).with(bundled_app_lock, "wb").
           and_raise(Errno::EACCES)
         expect { subject.lock }.
           to raise_error(Bundler::PermissionError, /Gemfile\.lock/)
@@ -24,7 +23,7 @@ RSpec.describe Bundler::Definition do
     context "when a temporary resource access issue occurs" do
       it "raises a TemporaryResourceError with explanation" do
         allow(File).to receive(:open).and_call_original
-        expect(File).to receive(:open).with("Gemfile.lock", "wb").
+        expect(File).to receive(:open).with(bundled_app_lock, "wb").
           and_raise(Errno::EAGAIN)
         expect { subject.lock }.
           to raise_error(Bundler::TemporaryResourceError, /temporarily unavailable/)