Reduce flakiness of a 'erase job trace' test

What does this MR do and why?

We should only assert on the page content, not the build attributes, which are low-level data, which should already be tested at a lower level.

This should hopefully reduce the flakiness of the test.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #391237 (closed)