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.
This makes the convention consistent with the DSL, and avoids
workarounds for conflicts with Lkt keywords: in Libadalang, the Null
token can stay Null, instead of null_tok (no API breakage needed).
Future work on Lkt will use this compiled type so that properties can
return precisely located errors on string literals.
Note that the exposition of this new compiled type to the OCaml and Java
APIs is incomplete, and is deferred to future developments.
