The problem was caused by the following scenario:
- range optimizer picks an index IDX1 which doesn't match the ORDER BY ...
  LIMIT clause.
- test_if_skip_sort_order() decides to switch to index IDX2 which matches
  the ORDER BY ... LIMIT.
- it runs SQL_SELECT::test_quick_select() for the second time to produce
  an quick select for IDX2.
- However, test_quick_select() would figure that full index scan on IDX1
  is still cheaper (its calculations ignore the LIMIT n).

Fixed this by
- passing force_quick_range=true to test_quick_select()
- in test_quick_select, don't consider full index scans if the mentioned
  parameter is true.

Numerous changes in .result files are caused by test_quick_select() being
run after "early/late NULLs filtering" feature has injected NOT NULL
condition.