[gardening] pkg/vm_snapshot_analysis/test/instruction_sizes_test flaky timing out

[dcharkes]

This test was flaking, and is now consistently timing out.

The commit on which this started happening is an analyzer commit, so that can't be the cause.

The tests

pkg/vm_snapshot_analysis/test/instruction_sizes_test Timeout (expected Pass)

are failing on configurations

unittest-asserts-debug-linux-x64

This test is doing quite a lot, maybe we should simply mark it as slow.

https://dart-current-results.web.app/#/filter=pkg/vm_snapshot_analysis/test/instruction_sizes_test&showAll

[dart-github-bot]

Summary: The instruction_sizes_test in pkg/vm_snapshot_analysis consistently times out on Linux x64, despite not being directly related to a recent analyzer change. Marking it as slow is suggested.

[dcharkes]

Duplicate of #56979