fix(mocks): $timeout#flush should not update time when empty

When $timeout#flush is called with a delay and no task can be flushed within that
delay, the current time should not be updated as that gets the mock into an inconsistent
state.

BREAKING CHANGE: if a tests was written around the buggy behavior the delays might be off now

This would typically not be a problem, but because of the previous breaking change in
$timeout.flush, the combination of two might be confusing and that's why we are documenting
it.

Old behavior:

```
doSomething(); //schedules task to execute in 500ms from now
doOtherStuff(); //schedules task to execute in 600ms from now

try {
  $timeout.flush(300); // throws "no task to be flushed" exception
} catch(e) {};
$time.flush(200); //flushes only doSomething() task
```

New behavior:

```
doSomething(); //schedules task to execute in 500ms from now
doOtherStuff(); //schedules task to execute in 600ms from now

try {
  $timeout.flush(300); // throws "no task to be flushed" exception
} catch(e) {};
$time.flush(200); // throws "no task to be flushed" exception again
                  // because previous exception didn't move the time forward
```

Fixed test:

```
doSomething(); //schedules task to execute in 500ms from now
doOtherStuff(); //schedules task to execute in 600ms from now

try {
  $timeout.flush(300); // throws "no task to be flushed" exception
} catch(e) {};
$time.flush(500); // flushes only doSomething() task
```