Bundler 2: Add test for bundler dependency

On v1, gems with a new version with a dependency on bundler 2 would fail to update,
this has since been fixed as demonstrated by this test.

bundler/spec/dependabot/bundler/file_updater_spec.rb

@@ -1596,6 +1596,33 @@
       end
     end
 
+    context "for a gem that depends on bundler" do
+      subject(:updated_gemfile) do
+        updated_files.find { |f| f.name == "Gemfile" }
+      end
+
+      let(:dependency_files) { bundler_project_dependency_files("guard_bundler") }
+      let(:dependency_name) { "guard-bundler" }
+      let(:dependency_version) { "3.0.0" }
+      let(:dependency_previous_version) { "2.2.1" }
+      let(:requirements) do
+        [{
+          file: "Gemfile",
+          requirement: "~> 2.2.1",
+          groups: [],
+          source: nil
+        }]
+      end
+
+      it "raises an error", :bundler_v1_only do
+        expect { updated_gemfile }.to raise_error(/Bundler could not find compatible versions for gem "bundler"/)
+      end
+
+      it "returns the latest version", :bundler_v2_only do
+        expect(updated_gemfile.content).to include("\"guard-bundler\", \"~> 2.2.1\"")
+      end
+    end
+
     context "vendoring" do
       let(:project_name) { "vendored_gems" }
       let(:repo_contents_path) { build_tmp_repo(project_name) }

bundler/spec/fixtures/projects/bundler1/guard_bundler/Gemfile

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gem "guard-bundler", "~> 2.2.1"

bundler/spec/fixtures/projects/bundler1/guard_bundler/Gemfile.lock

@@ -0,0 +1,46 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    coderay (1.1.3)
+    ffi (1.14.2)
+    formatador (0.2.5)
+    guard (2.16.2)
+      formatador (>= 0.2.4)
+      listen (>= 2.7, < 4.0)
+      lumberjack (>= 1.0.12, < 2.0)
+      nenv (~> 0.1)
+      notiffany (~> 0.0)
+      pry (>= 0.9.12)
+      shellany (~> 0.0)
+      thor (>= 0.18.1)
+    guard-bundler (2.2.1)
+      bundler (>= 1.3.0, < 3)
+      guard (~> 2.2)
+      guard-compat (~> 1.1)
+    guard-compat (1.2.1)
+    listen (3.4.1)
+      rb-fsevent (~> 0.10, >= 0.10.3)
+      rb-inotify (~> 0.9, >= 0.9.10)
+    lumberjack (1.2.8)
+    method_source (1.0.0)
+    nenv (0.3.0)
+    notiffany (0.1.3)
+      nenv (~> 0.1)
+      shellany (~> 0.0)
+    pry (0.14.0)
+      coderay (~> 1.1)
+      method_source (~> 1.0)
+    rb-fsevent (0.10.4)
+    rb-inotify (0.10.1)
+      ffi (~> 1.0)
+    shellany (0.0.1)
+    thor (1.1.0)
+
+PLATFORMS
+  ruby
+
+DEPENDENCIES
+  guard-bundler (= 2.2.1)
+
+BUNDLED WITH
+   2.1.4