* fix: add Promise<void> return type for expectation

Since 0.6.0 promises and async/await are supported and tested, but the return type of the expectation function did not yet reflect this change.

* fix SecurityError when running tests

Jest fails the tests with “SecurityError: localStorage is not available for opaque origins”. Setting the Jest configuration option "testURL" to "http://localhost/" fixes this (also see: jsdom/jsdom#2304 (comment)).
It’s probably a good idea to upgrade dependencies and remove this option afterwards, since with the current release of Jest this option is no longer required (it defaults to "http://localhost/“ instead of “about:blank”).