Actions: tolerate long ${{ ... }} expressions

[smowton]

These broke above about 300 characters because the regex in getADelimitedExpression contained an alternation inside a quantifier that the Java regex engine couldn't work out could never backtrack in practice, due to the mutual exclusivity of the alternatives with both one another and the suffix. Combined with its usage of the call stack for backtracking, at four frames per character that meant an overflow occurred quite quickly.

Fixed by making the quantifier possessive (*+), so it never backtracks once an iteration is satisfied.

I move the existing library tests into a subdirectory to avoid a new library test changing a wide variety of results, particularly ones that expose a ~100kb test string.