Split plot-smoke tests into parametrized cases

[ecomodeller]

Problem

A handful of plot tests are slow and provide poor diagnostic value. They stack many plot.scatter(...) calls into a single test body and end with assert True, so a failing kwarg combination requires manual bisection to locate.

Worst offenders (durations from a recent local run):

• tests/test_multimodelcompare.py::test_mm_scatter — 4.12 s, 12 scatter calls, two real assertions
• tests/test_multivariable_compare.py::test_mv_mm_scatter — 1.92 s, same shape
• tests/test_multimodelcompare.py::test_custom_metric_skilltable_mm_scatter — 0.39 s, same shape

Together they're ~14 % of total suite runtime (45 s).

Why the current shape is bad

• No useful failure signal. 12 calls in one test → you only know "something broke," not which kwarg combination.
• Discourages adding coverage. Every new scatter kwarg makes the test slower instead of expanding the matrix.
• Hides regressions. A change that breaks one combo but not the others still surfaces as one red dot.

Proposed fix

Convert each into @pytest.mark.parametrize over the kwarg variations. Each combination becomes its own test case with its own name and its own assertion. Same coverage, parallelisable, individually attributable failures, sub-second per variant.

Drop the trailing assert True — if the test only checks "didn't crash," a parametrize with no body still does that.

Scope

Plot-smoke tests only — not the tests that already have real assertions per call.