fix: Add Promise<void> return type for expectation (#26)

* 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”).

package.json

@@ -78,6 +78,7 @@
     "transform": {
       "^.+\\.(js|jsx|ts|tsx)$": "babel-jest"
     },
-    "testRegex": "/src/.*\\.spec\\.(js|ts|tsx)$"
+    "testRegex": "/src/.*\\.spec\\.(js|ts|tsx)$",
+    "testURL": "http://localhost/"
   }
 }

src/index.d.ts

@@ -6,4 +6,4 @@
  * @param  interval  Number  Wait-between-retries interval, 50ms by default
  * @return  Promise  Promise to return a callback result
  */
-export default function waitForExpect(expectation: () => void, timeout?: number, interval?: number): any;
+export default function waitForExpect(expectation: () => void | Promise<void>, timeout?: number, interval?: number): any;

src/index.ts

@@ -14,7 +14,7 @@ const defaults = {
  * @return  Promise  Promise to return a callback result
  */
 const waitForExpect = function waitForExpect(
-  expectation: () => void,
+  expectation: () => void | Promise<void>,
   timeout = defaults.timeout,
   interval = defaults.interval
 ) {