Specs: Optimize `associate_domain_service_spec` to use `let_it_be`

What does this MR do and why?

Related to work done in:

• !108462 (closed)
• &7582

Around 10s takes to start testing environment, so this is over 2x faster.

Now:

Finished in 14.73 seconds (files took 0.90135 seconds to load) 8 examples, 0 failures [TEST PROF INFO] Time spent in factories: 00:01.704 (10.82% of total time)

Before:

Finished in 21.04 seconds (files took 1.06 seconds to load) 8 examples, 0 failures [TEST PROF INFO] Time spent in factories: 00:07.054 (33.5% of total time)

MR acceptance checklist

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