Fix incorrect collapser source being generated due to unicode collapse points

Fix incorrect collapser source being generated due to unicode collapse points

I am not sure how nobody but abraxxa ran into this - the missing escapes have
been used by a ton of people for over a year. In any case the issue is real,
and fixed by trivial {}-escaping.

Add an extra stresstest envvar to make sure this does not regress in the
future (the envvar will be picked up and set by POISON_ENV on travis)

(cherry pick of 2fdeef65)