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.
(cherry picked from commit e1c4fe5804)
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).
This test fails sometimes because it makes no sense at all. There is no
information about rewriting context being equivalent when closing and
reopening a new one (see doc for 'langkit.rewriting.start_rewriting').
