Refine Spec Project#last_activity_date

Created by: jhund

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 (closed)

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 (closed)

/cc @jvanbaarsen