[CI] Register PIC unit tests and refresh stale fixtures

[morluto]

Motivation

PIC already has unit tests covering cache operations, eviction, scheduling, and model-specific paths, but they live outside the registered CPU test tree. Consequently, the normal CI test selection does not run them and regressions can land without exercising PIC behavior.

Before

python/sglang/test/srt/pic
             |
             v
      outside registry
             |
             v
      not selected by CI

After

test/registered/unit/pic
             |
             v
       register_cpu_ci
             |
             v
   24 files -> 63 PIC tests

Modifications

• Move the existing PIC tests into the registered unit-test tree.
• Register the suite for CPU CI.
• Update stale fixtures and assertions to match the current PIC interfaces.
• Preserve the existing test coverage rather than replacing it with a new parallel suite.

Accuracy Tests

Not applicable. This PR changes test registration and fixtures, not inference behavior.

Speed Tests and Profiling

Not applicable.

Test Plan

The registered PIC suite passes:

63 passed

The same suite initially produced 22 failures, 41 passes, and 1 skip before the stale fixtures were updated.

The final suite was also reproduced on a clean DigitalOcean Linux environment at commit 67aec6e9f9513787f196e2e3d97a777118b4f420.

Closes #3.