Add spec for warning messages when cache location is invalid

This PR suppresses the following warnings that are displayed
when running tests, and makes them executable test code.

```console
% cd path/to/rubocop
% bundle exec rspec ./spec/rubocop/result_cache_spec.rb:278
Run options: include {:focus=>true,
:locations=>{"./spec/rubocop/result_cache_spec.rb"=>[278]}}

Randomized with seed 7133
Couldn't create cache directory. Continuing without cache.
  Permission denied
.Couldn't create cache directory. Continuing without cache.
  Read-only file system
.

Finished in 0.20314 seconds (files took 1.93 seconds to load)
2 examples, 0 failures

Randomized with seed 7133
```

spec/rubocop/result_cache_spec.rb

@@ -271,19 +271,33 @@ def abs(path)
       end
     end
 
-    shared_examples 'invalid cache location' do |error|
+    shared_examples 'invalid cache location' do |error, message|
+      before do
+        $stderr = StringIO.new
+      end
+
       it 'doesn\'t raise an exception' do
         expect(FileUtils).to receive(:mkdir_p).with(start_with(cache_root))
                                               .and_raise(error)
         expect { cache.save([]) }.not_to raise_error
+        expect($stderr.string).to eq(<<~WARN)
+          Couldn't create cache directory. Continuing without cache.
+            #{message}
+        WARN
+      end
+
+      after do
+        $stderr = STDERR
       end
     end
 
     context 'when the @path is not writable' do
       let(:cache_root) { '/permission_denied_dir' }
 
-      it_behaves_like 'invalid cache location', Errno::EACCES
-      it_behaves_like 'invalid cache location', Errno::EROFS
+      it_behaves_like 'invalid cache location',
+                      Errno::EACCES, 'Permission denied'
+      it_behaves_like 'invalid cache location',
+                      Errno::EROFS, 'Read-only file system'
     end
   end