chore(bigquery): remove duplicate test dependencies

[plamut]

Fixes #9502.

This PR removes redundancy in BigQuery's noxfile that results in test_utils being installed twice.

(modifying the noxfile directly, as it is not generated by the synth tool in BigQuery)

How to test

• Run the following nox sessions: snippets and system. You do not have to wait for them to complete, just make sure the setup phase is completed and the relevant tests start running.

Actual result (before the fix):
In both cases test_utils are installed twice. In the output:

...
nox > pip install -e ../test_utils
...
nox > pip install -e ../test_utils

Expected result (after the fix):
test_utils are only installed once in a session.

[tseaver]

I would argue that dropping test_utils from LOCAL_DEPS would be clearer, if there are segments which don't actually need them.

[plamut]

@tseaver test_utils were actually moved to LOCAL_DEPS based on an earlier suggestion. :) All test suites need them (unit, system, snippets), although they seem redundant for the lint and docs nox sessions.

I can demote test_utils from LOCAL_DEPS to avoid that redundancy.

[plamut]

The Cloud Build test failure is unrelated, currently occurring on all PRs, merging.