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?

#7223

/cc @jvanbaarsen