ci: fix global.AbortController test coverage

test/fetch.js

@@ -7,6 +7,16 @@ const clock = new Clock()
 t.teardown(clock.enter())
 clock.advance(1)
 
+// if we're on a version that *doesn't* have a native AbortController,
+// put the polyfill in there to start with, so LRU covers both cases.
+global.AbortController = global.AbortController || Object.assign(
+  class AbortController {
+    constructor () { this.signal = new global.AbortController.AbortSignal }
+    abort () { this.signal.aborted = true }
+  },
+  { AbortSignal: class AbortSignal { constructor () { this.aborted = false }}}
+)
+
 const LRU = require('../')
 const c = new LRU({
   fetchMethod: fn,