test(markers): add missing datalad-core marker "integration"

Tests are executd with `pytest --strict-marker` now to catch undesired situations due to mistyped markers. DataLad (core) implements many of its old test decorators as markers, but does not declare them. This change add a missing one.

datalad#692

pyproject.toml

@@ -4,10 +4,13 @@ requires = ["setuptools >= 43.0.0", "wheel"]
 [tool.pytest.ini_options]
 addopts = "--strict-markers"
 markers = [
+    # datalad-next custom markers
+    "skip_if_no_network",
+    # (implicitely) used markers from datalad-core, which are not declared there
+    "integration",
     "network",
     "serve_path_via_http",
     "skip_if_adjusted_branch",
-    "skip_if_no_network",
     "skip_if_on_windows",
     "skip_if_root",
     "skip_ssh",