[rb] fix Ruby tests failing because of alerts in Firefox (#17294)

* fix Ruby tests failing because of alerts in Firefox

Seems that Firefox behavior with alerts was changed. Without "unhandledPromptBehavior" capability, command `wait_for_alert` is always failing in FireFox.

Author: asolntsev

rb/spec/integration/selenium/webdriver/spec_support/test_environment.rb

@@ -324,6 +324,7 @@ def firefox_options(args: [], **opts)
           opts[:browser_version] = 'stable' if WebDriver::Platform.windows?
           opts[:web_socket_url] = true if ENV['WEBDRIVER_BIDI'] && !opts.key?(:web_socket_url)
           opts[:binary] ||= rlocation(ENV['FIREFOX_BINARY']) if ENV.key?('FIREFOX_BINARY')
+          opts[:unhandled_prompt_behavior] ||= 'ignore'
           args << '--headless' if ENV['HEADLESS']
           WebDriver::Options.firefox(args: args, **opts)
         end