infra: use pytest fixtures in batch transform integ tests to train and upload to S3 only once

[laurenyu]

Issue #, if available:

Description of changes:
These integ tests are tested batch transform jobs, not training jobs, so there's no need to start over with an estimator in every test. Also, it's the same input data getting uploaded to S3 for each MXNet test, so there's also no need to do that every time.

I tried not to modify the actual content of any test.

Testing done:
tox -e black-format

Merge Checklist

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General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have passed the region in to any/all clients that I've initialized as part of this change.
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)
• I have used unique_name_from_base to create resource names in integ tests (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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