Fix fragile test (`AssociationProxyTest#test_save_on_parent_saves_children`)

If we run only following tests:

- test/cases/scoping/default_scoping_test.rb
- test/cases/associations_test.rb

```
$ cat Rakefile.test
require "rake/testtask"

ENV["ARCONN"] = "postgresql"

Rake::TestTask.new do |t|
  t.libs << "test"
  t.test_files = %w(
    test/cases/scoping/default_scoping_test.rb
    test/cases/associations_test.rb
  )
end
```

a test will fail:

```
$ bundle exec rake test -f Rakefile.test
/app/activesupport/lib/active_support/core_ext/enumerable.rb:20: warning: method redefined; discarding old sum
Using postgresql
Run options: --seed 11830

# Running:

.........................................................................................F................

Finished in 6.939055s, 15.2759 runs/s, 27.9577 assertions/s.

  1) Failure:
AssociationProxyTest#test_save_on_parent_saves_children [/app/activerecord/test/cases/associations_test.rb:185]:
Expected: 1
  Actual: 2

106 runs, 194 assertions, 1 failures, 0 errors, 0 skips
rake aborted!
Command failed with status (1)
/usr/local/bin/bundle:22:in `load'
/usr/local/bin/bundle:22:in `<main>'
Tasks: TOP => test
(See full trace by running task with --trace)
```

In #28083, change `self.use_transactional_tests` to `false`
but we forget to clean-up fixture.
However we don't have to disable transaction except a few tests.

0 files changed, 0 insertions, 0 deletions