Add test to check that it warns when using the deprecated config

spec/erb_lint/cli_spec.rb

@@ -103,6 +103,20 @@ def run(processed_source)
       end
     end
 
+    context "with deprecated config file" do
+      let(:deprecated_config_filename) { ".erb-lint.yml" }
+      let(:config_file_content) { "------\nEnableDefaultLinters: true\n" }
+
+      before do
+        FileUtils.mkdir_p(File.dirname(deprecated_config_filename))
+        File.write(deprecated_config_filename, config_file_content)
+      end
+
+      it "shows a warning but loads the deprecated config file" do
+        expect { cli.run(args) }.to(output(/#{Regexp.escape(deprecated_config_filename)} is deprecated/).to_stderr)
+      end
+    end
+
     context "with --disable-inline-configs" do
       module ERBLint
         module Linters