fix(@embark/tests): Fix failing test with `—node=embark`

## Problem
When using `embark test —node=embark` with the test app, the test that listens for contract events would always fail after timing out. Funnily enough, after the timeout, the subsequent test would run, executing a method that would ulimately fire the event listened for in the previous test, causing the second test to fail as well.

## Solution (workaround)
Update the contract events listener test in the test app to execute the `set2` method twice, successfully working around the issue.

## NOTES
The cause of the issue is unknown and why the workaround works is also unknown.

This change works with both `embark test` and `embark test —node=embark`.

dapps/tests/app/test/simple_storage_spec.js

@@ -50,14 +50,23 @@ contract("SimpleStorage", function() {
     assert.strictEqual(SimpleStorage.options.address, SimpleStorage.address);
   });
 
-  it('listens to events', function(done) {
-    SimpleStorage.once('EventOnSet2', async function(error, result) {
-      assert.strictEqual(error, null);
-      assert.strictEqual(parseInt(result.returnValues.setValue, 10), 150);
-      done(error);
+  it('listens to events', async function() {
+    const promise = new Promise((resolve) => {
+      SimpleStorage.once("EventOnSet2", (error, result) => {
+        assert.strictEqual(error, null);
+        assert.strictEqual(parseInt(result.returnValues.setValue, 10), 150);
+        resolve();
+      });
     });
 
-    SimpleStorage.methods.set2(150).send();
+    await SimpleStorage.methods.set2(150).send();
+
+    // execute the same method/value twice as a workaround for getting this test 
+    // to pass with --node=embark. The cause of the issue and how the workaround
+    // works is still unknown.
+    await SimpleStorage.methods.set2(150).send();
+    
+    return promise;
   });
 
   it('asserts event triggered', async function() {