Skip url_helpers instead of caching, speed up integration tests

We shouldn't cache if it's not absolutely necessary. Removes
route caching and instead skips using the `url_helpers` is the
integration test session doesn't require it. Benchmark ips on
integration and controller index method tests below.

Without any caching or changes to `#url_helpers`:
```
Calculating -------------------------------------
INDEX: Integration Test
                        71.000  i/100ms
INDEX: Functional Test
                        99.000  i/100ms
-------------------------------------------------
INDEX: Integration Test
                        728.878  (± 8.0%) i/s -      3.692k
INDEX: Functional Test
                          1.015k (± 6.7%) i/s -      5.148k

Comparison:
INDEX: Functional Test:     1015.4 i/s
INDEX: Integration Test:      728.9 i/s - 1.39x slower
```

With caching on `#url_helpers`:
```
Calculating -------------------------------------
INDEX: Integration Test
                        74.000  i/100ms
INDEX: Functional Test
                        99.000  i/100ms
-------------------------------------------------
INDEX: Integration Test
                        752.377  (± 6.9%) i/s -      3.774k
INDEX: Functional Test
                          1.021k (± 6.7%) i/s -      5.148k

Comparison:
INDEX: Functional Test:     1021.1 i/s
INDEX: Integration Test:      752.4 i/s - 1.36x slower
```

Afer removing the caching and bypassing the `url_helpers` when not
necessary in the session:
```
Calculating -------------------------------------
INDEX: Integration Test
                        87.000  i/100ms
INDEX: Functional Test
                        97.000  i/100ms
-------------------------------------------------
INDEX: Integration Test
                        828.433  (± 6.4%) i/s -      4.176k
INDEX: Functional Test
                        926.763  (± 7.2%) i/s -      4.656k

Comparison:
INDEX: Functional Test:      926.8 i/s
INDEX: Integration Test:      828.4 i/s - 1.12x slower
```

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -409,14 +409,6 @@ def #{name}
       end
 
       def url_helpers(supports_path = true)
-        if supports_path
-          @url_helpers_with_paths ||= generate_url_helpers(supports_path)
-        else
-          @url_helpers_without_paths ||= generate_url_helpers(supports_path)
-        end
-      end
-
-      def generate_url_helpers(supports_path)
         routes = self
 
         Module.new do

actionpack/lib/action_dispatch/testing/integration.rb

@@ -222,15 +222,6 @@ def initialize(app)
         super()
         @app = app
 
-        # If the app is a Rails app, make url_helpers available on the session
-        # This makes app.url_for and app.foo_path available in the console
-        if app.respond_to?(:routes)
-          singleton_class.class_eval do
-            include app.routes.url_helpers
-            include app.routes.mounted_helpers
-          end
-        end
-
         reset!
       end
 
@@ -396,6 +387,8 @@ def build_full_uri(path, env)
     module Runner
       include ActionDispatch::Assertions
 
+      APP_SESSIONS = {}
+
       def app
         @app ||= nil
       end
@@ -403,7 +396,19 @@ def app
       # Reset the current session. This is useful for testing multiple sessions
       # in a single test case.
       def reset!
-        @integration_session = Integration::Session.new(app)
+        @integration_session = create_session(app)
+      end
+
+      def create_session(app)
+        klass = APP_SESSIONS[app] ||= Class.new(Integration::Session) {
+          # If the app is a Rails app, make url_helpers available on the session
+          # This makes app.url_for and app.foo_path available in the console
+          if app.respond_to?(:routes)
+            include app.routes.url_helpers
+            include app.routes.mounted_helpers
+          end
+        }
+        klass.new(app)
       end
 
       def remove! # :nodoc: