Problem: Not enough folding code is tested.
Solution: Add more test cases. (Yegappan Lakshmanan, closesvim/vim#8046)
5c504f680e
Reorder test_fold.vim to match upstream.
Cherry-pick Test_fold_expr_error() from patch 8.2.0633.
Cherry-pick syntax feature check from patch 8.2.1432.
Cherry-pick a delete() call from patch 8.2.2112.