I think the development of Tree-sitter modes would benefit from having a
regression test suite that ensures that bug fixes for specific cases
don't regress in the future.

As a start, I've attached to this mail some ERT-based tests for the
indentation feature of c-ts-mode.  I've used the convenient feature of
"erts" files to create the tests, which I think make them readable and
maintainable.

The tests are skipped if the Emacs build does not have Tree-sitter
enabled, or if the corresponding grammar is not installed.  Perhaps it
would be a good idea to install these Tree-sitter dependencies in the
container images used for EMBA (https://emba.gnu.org/emacs), so that
we're able to run Tree-sitter tests in continuous integration, and even
play with different versions of Tree-sitter or the Tree-sitter grammars
to detect incompatibilities.  I'll leave this decision to the people in
charge of EMBA.

Thanks.