Refactor assert revert helper to encapsulate promises (#628)

test/helpers/assertRevert.js

@@ -1,3 +1,9 @@
-module.exports = function (error) {
-  assert.isAbove(error.message.search('revert'), -1, 'Error containing "revert" must be returned');
+export default async promise => {
+  try {
+    await promise;
+    assert.fail('Expected revert not received');
+  } catch (error) {
+    const revertFound = error.message.search('revert') >= 0;
+    assert(revertFound, `Expected "revert", got ${error} instead`);
+  }
 };