Strict folding for folding.el + failsafe for other modes

Hey,

(Follow-up from https://github.com/mlf176f2/fold-dwim-org/issues/1 (sorry if there is a better way of doing this, not familiar enough with guthub yet).)

The function should-fold-p was still not called out of hs-minor-mode. I also improved the test for folding.el, it turns out there was a ready-to-use function for our test.

I also added the failsafe code I was suggesting for TeX-fold, outline-minor-mode and nxml, so that "strict" doesn't mean "disabled" for these modes.