Don't call register on custom drivers

It's possible for developers toadd a custom driver and then call it
using `driven_by`. Because we were only skipping `register` for
`:rack_test` that meant any custom driver would attempt to be registered
as well.

The three listed here are special because Rails registers them with
special options. If you're registering your own custom driver then you
don't want to separately register that driver.

Fixes #29688

actionpack/lib/action_dispatch/system_testing/driver.rb

@@ -9,14 +9,14 @@ def initialize(name, **options)
       end
 
       def use
-        register unless rack_test?
+        register if registerable?
 
         setup
       end
 
       private
-        def rack_test?
-          @name == :rack_test
+        def registerable?
+          [:selenium, :poltergeist, :webkit].include?(@name)
         end
 
         def register

actionpack/test/dispatch/system_testing/driver_test.rb

@@ -29,7 +29,7 @@ class DriverTest < ActiveSupport::TestCase
     assert_equal ({ skip_image_loading: true }), driver.instance_variable_get(:@options)
   end
 
-  test "rack_test? returns false if driver is poltergeist" do
-    assert_not ActionDispatch::SystemTesting::Driver.new(:poltergeist).send(:rack_test?)
+  test "registerable? returns false if driver is rack_test" do
+    assert_not ActionDispatch::SystemTesting::Driver.new(:rack_test).send(:registerable?)
   end
 end