test: close pools on tests cleanup

This avoids weird errors as pools are collected by gc in broken env.

tests/conftest.py

@@ -169,3 +169,11 @@ def erepo(repo_dir):
         yield repo
     finally:
         repo.tearDown()
+
+
+@pytest.fixture(scope="session", autouse=True)
+def _close_pools():
+    from dvc.remote.pool import close_pools
+
+    yield
+    close_pools()