Speed up GOC3 smoke test in CI

[michel2323]

Problem

The GOC3 testset builds and solves a ~140k-variable problem (the C3E4N00073D1_scenario_303 case). On CPU, the UMFPACK factorization dominates: a single solve takes ~390s (example run).

Two things made this worse than necessary:

1. It ran once per CONFIG — i.e. 4x on CPU (Float64/Float32 × nothing/CPU()) + 2x on GPU. In exasolve, both nothing and CPU() use UMFPACK, so the same ~390s CPU solve repeated 4 times (~1560s of CI).
2. It asserts nothing on the result — sc_tests has no @tests; it's a build-and-step smoke check. The (T, backend) variants of the same CPU solve added no real coverage, and at max_iter=5 the problem never gets near convergence anyway (descent-direction norm ~1e11).

Change

• Hoist the GOC3 testset out of the per-CONFIG loop: run it once on CPU (Float64, nothing) and once on GPU (Float64, CUDABackend()) when a GPU is present.
• Drop max_iter from 5 to 1 in sc_tests — one step still exercises model build + KKT assembly + a factorization, which is all the smoke test checks.

Effect

CPU GOC3 time drops from ~4 × 390s to a single max_iter=1 solve; GPU from 2 solves to 1. The full-size problem and the contingency path are still built and stepped, so no meaningful coverage is lost.

If CI is still too slow afterward, the next lever is goc3_model(...; include_ctg=false) (the contingency constraints are what inflate the case to ~140k vars) — left out here to preserve contingency-path coverage.

[frapac]

LGTM!

[michel2323]

Removed some more redundancies

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.86%. Comparing base (ad165e4) to head (c2e4300).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #48   +/-   ##
=======================================
  Coverage   88.86%   88.86%           
=======================================
  Files           9        9           
  Lines        1275     1275           
=======================================
  Hits         1133     1133           
  Misses        142      142           

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[michel2323]

Down to 90m from 190m.