build: Include tests in source distributions

Include test files in the source distribution, to make it possible
to run them while repackaging from sources.  This permits Linux
distributions to use signed PyPI release artficats rather than having
to use the git repository directly.

Fixes narwhals-dev#2284

Author: mgorny

pyproject.toml

@@ -110,7 +110,6 @@ local-dev = [
 exclude = [
   "/.*",
   "/docs",
-  "/tests",
   "/tpch",
   "/utils",
   ".gitignore",