test: tighten patchwork assertion to match corrected plan (#87 Copilot review)

Plan #87 was corrected (via PR #87 Copilot review) to assert patchwork
specifically for the 4-predictor iris default-plot case, rather than the
loose patchwork||ggplot form that wouldn't catch a regression to a bare
list (issue #80).  Align the implementation test to match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ehrlinger

tests/testthat/test_gg_variable.R

@@ -383,7 +383,10 @@ test_that("gg_variable.randomForest classification: plot returns patchwork for a
   rf <- randomForest::randomForest(Species ~ ., data = iris, ntree = 50L)
   gg <- gg_variable(rf)
   p  <- plot(gg)
-  expect_true(inherits(p, "patchwork") || inherits(p, "ggplot"))
+  # iris has 4 predictors so the no-xvar default assembles a multi-panel
+  # patchwork; assert patchwork specifically to catch regressions to a bare
+  # list (#80).
+  expect_s3_class(p, "patchwork")
 })
 
 test_that("gg_variable.randomForest classification: layer_data works on single-xvar plot", {