test: add test to reproduce described defect

cargo/spec/dependabot/cargo/update_checker/version_resolver_spec.rb

@@ -533,5 +533,23 @@
       let(:dependency_files) { unprepared_dependency_files }
       it { is_expected.to eq(Gem::Version.new("0.1.80")) }
     end
+
+    context "when multiple packages have a version conflict with one another" do
+      let(:dependency_name) { "ructe" }
+      let(:dependency_version) { "0b8acfe5eea15713bc56c156f974fa05967d0353" }
+      let(:string_req) { nil }
+      let(:source) do
+        {
+          type: "git",
+          url: "https://github.com/kaj/ructe",
+          branch: nil,
+          ref: nil
+        }
+      end
+      let(:dependency_files) { project_dependency_files("version_conflict") }
+      let(:unprepared_dependency_files) { project_dependency_files("version_conflict") }
+
+      specify { expect(subject).to be_nil }
+    end
   end
 end