test: Use the deadline cloud test fixtures for xa11y tests.

[karthikbekalp]

What was the problem/requirement? (What/Why)

DCC submitter UI tests need common infrastructure for running against an offline Deadline Cloud backend, interacting with the submitter through accessibility APIs, and validating exported job bundles and rendered images.

This infrastructure was implemented locally in the Cinema 4D integration tests, which made it difficult for other DCC integrations to reuse and maintain.

What was the solution? (How)

This is part 2 of the effort.

We have already merged and released changes to deadline-cloud-test-fixtures. aws-deadline/deadline-cloud-test-fixtures#301

This just removes duplicate code in Cinema 4D and re-uses the deadline cloud test fixtures code.

What is the impact of this change?

More reusability. No improvement in any other aspects.

How was this change tested?

Ran these tests locally on Mac and Windows.

test/integ_xa11y/test_cinema4d.py::test_integ[cube] 
[gw0] [100%] PASSED test/integ_xa11y/test_cinema4d.py::test_integ[cube] 

================================================ slowest 5 durations =================================================
30.86s call     test/integ_xa11y/test_cinema4d.py::test_integ[cube]
0.25s setup    test/integ_xa11y/test_cinema4d.py::test_integ[cube]
0.00s teardown test/integ_xa11y/test_cinema4d.py::test_integ[cube]
================================================= 1 passed in 31.76s =================================================

Was this change documented?

No customer facing impact.

Is this a breaking change?

No

If so, then please describe the changes that users of this package must make to update their scripts, or Python applications. Also,
please ensure that the title of your commit follows our conventional commit guidelines in
CONTRIBUTING.md for breaking changes.
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