perf(core/a11y): start loading axe earlier

[sidvishnoi]

This will start fetching the axe script earlier, giving some performance improvements. Also, should help in avoiding jasmine timeouts that we often get in CI [^1].

Extracted from https://github.com/w3c/respec/pull/3319

[^1]

  Core — a11y
    ✖ allows overriding options (slow: 5.003 secs)
      Firefox 86.0 (Ubuntu 0.0.0)
    Error: Timeout - Async function did not complete within 5000ms (set by jasmine.DEFAULT_TIMEOUT_INTERVAL) in node_modules/jasmine-core/lib/jasmine-core/jasmine.js (line 7048)
    <Jasmine>