Make `driven_by` overridable

[mtsmfm]

Sometimes we want to use rack_test partially instead of selenium for test speed:

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  driven_by :selenium, using: :chrome, screen_size: [1400, 1400], options: {url: "http://chrome:4444/wd/hub"}
end

class WithJavascriptTest < ApplicationSystemTestCase
end

class WithoutJavascriptTest < ApplicationSystemTestCase
  driven_by :rack_test
end

In the abobe case, WithoutJavascriptTest uses selenium because
SystemTestCase calls superclass' driver on #initialize (self.class.superclass.driver.use).

Using class_attribute can handle inherited driven_by.

[rails-bot]

r? @eileencodes

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[eileencodes]

I was confused why this test was necessary when the test before works with rack_test. Can you rename it to OverrideSeleniumSubclassToRackTestTest so it's clearer?

[mtsmfm]

@eileencodes I renamed the test class!

[eileencodes]

Backported to 5-1-stable in 41412ef