Fix pub/sub unit test

[liamtoozer]

What is the context of this PR?

A recent PR was raised to remove a failing Pub/Sub unit test (#1504), which was asserting an API response exception from GCP directly. This was causing failures because GCP had changed their error message text.

Instead, we can just assert that the exception is raised in our function, and patch the pub/sub exception so that we don't query GCP directly

How to review

Check that the test passes and that it makes sense

Checklist

• New static content marked up for translation
• Newly defined schema content included in eq-translations repo

[Yuyuutsu]

Relook - changes look good, and works as expected

[petechd]

Not much we can do here, except the changes you've made. Unit test is relying on external APIs.