Transition all relevant testcases to it. In the process, port
docstrings/comments from test.py files to expected_concrete_syntax.lkt
(now test.lkt) sources and fix stylechecks issues there.
While it is currently quite easy to implement such a property in the DSL
directly (.mapcat on a String.array), an upcoming commit will introduce
an opaque String type, making it much less convenient (replace a simple
.mapcat with a recursive property). Since this feature is generally
useful, just add a builtin property for it.
TN: UA05-027
