Fix test ordering problem in transaction_leaking_spec and single-db test failure

What does this MR do and why?

I tried a bunch of different ways to fix the test ordering problem described in #382439 (closed) . It seemed that it was not enough to reset the for_model method in after do which makes sense because these stubs are already reset between tests. I needed to also call Setup again on the model because the incorrect host configuration was memoized on the model which is not reset between tests. Then I found this alone was not sufficient beause I noticed that the test is using for_model(ApplicationRecord) which is not actually ever being called in the initializer in normal code execution. It seems it's meant to be ActiveRecord::Base and we do not correctly clear whatever is cached between tests unless we use that consistently.

This problem and solution can be reproduced by running the specs like:

bin/rspec --format documentation --order defined spec/lib/gitlab/database/load_balancing/transaction_leaking_spec.rb spec/initializers/load_balancing_spec.rb

The other commit in this MR is from !103839 (closed) which fixes a test that is not meant to run in single-db and skips for the single-db jobs.

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Related to #382439 (closed)