test: add test harness with http and router tests

test/support.rb

@@ -0,0 +1,24 @@
+require 'prism'
+require 'stringio'
+
+module TestSupport
+  def env_for(method, path, headers: {}, body: nil, cookies: nil)
+    path_part, query = path.split('?', 2)
+    env = {
+      'REQUEST_METHOD' => method.to_s.upcase,
+      'PATH_INFO' => path_part,
+      'QUERY_STRING' => query.to_s,
+      'rack.input' => StringIO.new(body || ''),
+      'CONTENT_LENGTH' => (body ? body.bytesize.to_s : '0')
+    }
+    headers.each do |name, value|
+      if name.downcase == 'content-type'
+        env['CONTENT_TYPE'] = value
+      else
+        env["HTTP_#{name.upcase.tr('-', '_')}"] = value
+      end
+    end
+    env['HTTP_COOKIE'] = cookies if cookies
+    env
+  end
+end