[submodule] Update test/ptf to a Python 3.12+ compatible revision

[nicholasching]

Context / motivation
test/ptf is pinned at 7494366 from December 2022. That revision's ptf runner does import imp at module scope and uses imp.load_module in load_test_modules. The imp module was removed in Python 3.12, so on any modern distribution PTF fails before running a single test:

File "ptf", line 19, in <module>
    import imp
ModuleNotFoundError: No module named 'imp'

This affects every sai_test module rather than any particular feature. It was found while bringing up a Debian Trixie based test image, which ships Python 3.13.

What this change does

• Moves test/ptf from 7494366 to d587084, the current head of p4lang/ptf main. The change that matters is 771a452 ("Replace use of deprecated package imp with current importlib"), present in the new pin and absent from the old.

Scope / risk

• Submodule pointer only. No SAI source changes.
• The range spans roughly four years of upstream PTF work, including a uv/pyproject project layout, the nnpy to pynng transition, and scapy 2.7 support, so consumers pinning older Python or older scapy should check their environment.
• 771a452 is the minimum revision that unblocks Python 3.12+, if a smaller step is preferred.
• Validated behavior: the test image was rebuilt against the new pin and a 90-selector matrix completed with no infrastructure failures.

Dependencies
None functionally, but as a submodule bump this should be the last of the SAIVPP Phase 4 SAI changes to merge.

[azure-pipelines]

Azure Pipelines:
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[nicholasching]

Hi @kcudnik @tjchadaga, when you have a chance, could you please invoke azure pipelines to run checks. Thanks!