matrix-client-crypto.spec.js: replace callbacks with promises
matrix-client-crypto.spec.js: replace callbacks with promises

The pyramid of doom was getting unmanageable, not to mention the difficulty of
diagnosing why tests were failing, so replace the callbacks with promises.

krombel/matrix-js-sdk,williamboman/matrix-js-sdk,matrix-org/matrix-js-sdk,williamboman/matrix-js-sdk,matrix-org/matrix-js-sdk,krombel/matrix-js-sdk,williamboman/matrix-js-sdk,krombel/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk