fix(pivot-table): correct expected combination order in percent_total pruning test

buildGroupbyCombinations iterates rows as the outer loop and columns as
the inner loop, so when both the rows-collapsed and columns-collapsed
levels are pruned in (the percent_total case), the rows-collapsed
level pairs with every column prefix before the leaf row prefix does
-- matching the row/col-only pruning tests directly above it. The new
percent_total test had those two entries swapped; order carries no
functional meaning downstream (transformProps/PivotData key off each
row's own tags, not array position), so this fixes the expectation
rather than the (correct) production code.

Author: rusackas

superset-frontend/plugins/plugin-chart-pivot-table/test/plugin/utilities.test.ts

@@ -407,10 +407,13 @@ test('pruning: percent_total forces the grand-total level even with both totals
     rowSubTotals: false,
     showValuesAs: ShowValuesAsEnum.PERCENT_OF_TOTAL,
   });
+  // Rows is the outer loop and columns the inner loop (see
+  // buildGroupbyCombinations's flatMap), so the rows-collapsed level pairs
+  // with every column prefix before the leaf row prefix does.
   expect(combinations).toEqual([
     { rows: [], columns: [] },
-    { rows: ['row1', 'row2'], columns: [] },
     { rows: [], columns: ['col1', 'col2'] },
+    { rows: ['row1', 'row2'], columns: [] },
     { rows: ['row1', 'row2'], columns: ['col1', 'col2'] },
   ]);
 });