tests: stop re-importing pcapkit before every test (purge_modules dominates suite time) #1065

Is your feature request related to a problem? Please describe.
The #1062 audit found that 1,564 tests across 119 modules (243 call sites) call purge_modules(['pcapkit']) in setUp. That forces a fresh import of pcapkit (329 modules, about 0.67 s) before each of those tests. In the audit, disabling the purge cut pytest time on 192 modules from 1,714 s to 253 s (−85%).

I spot-checked one module on main: tests/protocols/application/test_http_unit.py gave 65 passed / 395 subtests in 77.38 s with the purge, and 0.29 s with it stubbed out.

Describe the solution you'd like
Re-import only where isolation is actually needed. Eight modules fail or hang without the purge: enum_builtin_parity, fields_collections, fields_field, pcapng_unit, dispatch_reachability, tcp_http_dispatch, test_support_helpers, and foundation/test_extraction (hangs). Candidate approaches:

• purge once per class in setUpClass;
• purge only in the modules that need it;
• find and fix the state those eight depend on.

The plain-unittest leg's cross-generation check (#981) must keep catching what it catches today.

Describe alternatives you've considered
Deleting tests: #1062 found that the genuinely redundant ones would save about 2 s.

Additional context
Raw data is in the #1062 audit (/tmp/audit1062/ on the audit host). Related: #1052.