fix: add `Lazy-load Axe::API::Run` to save boot time

[cbliard]

It saves 95% of load time when doing require "axe-rspec". On my tests, I went from 135ms to 8ms.

The time is actually mostly spent loading the virtus library. Deferring the loading of Axe::API::Run until needed does in turn defer the loading of virtus library.

This is to improve the time is takes to run a single rspec file which does not involve any a11y, but yet requires axe-rspec in the spec_helper.

No QA required.