Update mocha from v7 to v9

`mocha` has been updated to v9. The breaking changes made for v8 and
v9 don't affect us [1][2]. Most notably they include dropping support
for Node.js v10.

The "unhandled rejection" handler setup in the test helper was removed,
as this is now default behaviour in `mocha` as of v8.2.0 [3].

[1]: https://github.com/mochajs/mocha/releases/tag/v9.0.0
[2]: https://github.com/mochajs/mocha/releases/tag/v8.0.0
[3]: mochajs/mocha#2640 (comment)

package.json

@@ -36,7 +36,7 @@
     "ethereumjs-wallet": "^0.6.3",
     "jsdom": "^11.12.0",
     "jsdom-global": "^3.0.2",
-    "mocha": "^7.1.2",
+    "mocha": "^9.0.1",
     "polyfill-crypto.getrandomvalues": "^1.0.0",
     "sinon": "^7.2.7"
   },

test/helper.js

@@ -1,6 +1,3 @@
-// disallow promises from swallowing errors
-enableFailureOnUnhandledPromiseRejection()
-
 const getRandomValuesPoly = require('polyfill-crypto.getrandomvalues')
 
 //
@@ -24,33 +21,3 @@ if (!window.crypto) {
 if (!window.crypto.getRandomValues) {
   window.crypto.getRandomValues = getRandomValuesPoly
 }
-
-function enableFailureOnUnhandledPromiseRejection (...args) {
-  // modified from https://github.com/mochajs/mocha/issues/1926#issuecomment-180842722
-
-  // rethrow unhandledRejections
-  if (typeof process !== 'undefined') {
-    process.on('unhandledRejection', function (reason) {
-      throw reason
-    })
-  } else if (typeof window !== 'undefined') {
-    // 2016-02-01: No browsers support this natively, however bluebird, when.js,
-    // and probably other libraries do.
-    if (typeof window.addEventListener === 'function') {
-      window.addEventListener('unhandledrejection', function (evt) {
-        throw evt.detail.reason
-      })
-    } else {
-      const oldOHR = window.onunhandledrejection
-      window.onunhandledrejection = function (evt) {
-        if (typeof oldOHR === 'function') {
-          oldOHR.apply(this, args)
-        }
-        throw evt.detail.reason
-      }
-    }
-  } else if (typeof console !== 'undefined' &&
-      typeof (console.error || console.log) === 'function') {
-    (console.error || console.log)('Unhandled rejections will be ignored!')
-  }
-}