Test/assertRevert: Fix late return bug (#1123)

We now ensure that if an exception is thrown while awaiting the promise,
the exception _has_ to be a revert. We throw 'Expected revert not
received' only afterwards. This solves any problems with confusing the
word 'revert'.

Fix #775

test/helpers/assertRevert.js

 async function assertRevert (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`);
+    return;
   }
+  assert.fail('Expected revert not received');
 }
 
 module.exports = {