Add: Acceptance test for matching unresolved variable

Ref: https://dev.cfengine.com/issues/6519
In general, when matching an unexpanded var, we probably want to match it *anywhere in the text* so would want to test `".*\$\([a-zA-Z0-9_.]+\).*"` (and ISTR regcmp's regex is anchored as if with `^...$` in any case, so your use of these here is fatuous).