Increase auto-techsupport wait for Arista-720DT

[Xichen96]

Why I did it

Fixes #25079.

test_auto_techsupport is flaky on slow platforms because the test waits for a new techsupport tarball after triggering auto-techsupport. For Arista-720DT, recent 202511 nightly failures hit the same signature described in #25079:

AssertionError: New expected techsupport file was not generated

Evidence checked while investigating 720DT nightlies:

• Public issue Flaky: show_techsupport.test_auto_techsupport - Techsupport file not generated in time #25079 already tracks this failure class and lists Arista-720DT as affected.
• Kusto showed 720DT m0 failures on SONiC.20251110.35 for:
  • test_rate_limit_interval
  • test_max_limit[techsupport]
• The failing and passing 720DT runs used the same image commit (9d5a100af0), so this is not explained by an image update.
• We also checked mgmt commits in the fail/pass window and did not find a show_techsupport/loganalyzer/sanity change that would explain the difference.
• After deploy-mg, the two target 720DT cases passed 10x locally, so the issue remains flaky/state-dependent rather than always reproducible.

This PR follows the existing Nokia slow-platform handling by adding Arista-720DT to the slow techsupport platform list.

How I did it

Add x86_64-arista_720dt_48s to the platforms that use a 900s wait for the techsupport tarball to appear.

The default remains 600s for other platforms.

How to verify it

Ran syntax check:

python3 -m py_compile tests/show_techsupport/test_auto_techsupport.py

Manual 720DT validation during investigation:

./run_tests.sh -n testbed-bjw2-can-720dt-6 -i ../ansible/veos,../ansible/bjw2 -u -x -a False -t m0,any \
  -c "show_techsupport/test_auto_techsupport.py::TestAutoTechSupport::test_rate_limit_interval" \
  -c "show_techsupport/test_auto_techsupport.py::TestAutoTechSupport::test_max_limit[techsupport]"

Result after deploy-mg: 10 consecutive runs passed for the two target cases (20 test executions).

Description for the changelog

Increase auto-techsupport tarball wait time for Arista-720DT platforms.

[mssonicbld]

/azp run

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