Refine Spec Project#last_activity_date

### What does this MR do?

It fixes two issues with the spec for `Project#last_activity_date`:

* make timing requirement less stringent so that it doesn't fail on slow systems like Travis CI
* fix logical error, replace `project.last_activity_at` with `project.last_activity_date` since this is what's being tested in this spec.

For more details please see #7223

### Why was this MR needed?

Because Travis CI builds fail unnecessarily and the test has a logical error that renders the test useless.

### What are the relevant issue numbers / Feature requests?

Fixes #7223

/cc @jvanbaarsen
What do you think about the following syntax:
```ruby
date_difference = (project.last_activity_date - last_event.created_at).to_i.abs 
expect(date_difference).to be_within(2)
```

This complies with the new rspec syntax better.