Fixes interop issue between native promises and RSVP promises in test helper hooks

[drewlee]

For reasons I'm not too sure of, the interoperability between the RSVP promise which is utilized by nextTickPromise and the native Promise used at:

ember-test-helpers/addon-test-support/@ember/test-helpers/-internal/helper-hooks.ts

Line 62 in 6573383

return Promise.all(promises).then(() => {});

causes numerous component test failures throughout our test suites. Using RSVP's Promise.all instead, fixes the test failures.

[drewlee]

@scalvert @rwjblue if you could take a look. Thanks!