Extract dask and spark test into distributed test. (#8395)

- Move test files.
- Run spark and dask separately to prevent conflicts.
- Gather common code into the testing module.

tests/test_distributed/test_gpu_with_spark/conftest.py

@@ -0,0 +1,10 @@
+from typing import Sequence
+
+import pytest
+
+
+def pytest_collection_modifyitems(config: pytest.Config, items: Sequence) -> None:
+    # mark dask tests as `mgpu`.
+    mgpu_mark = pytest.mark.mgpu
+    for item in items:
+        item.add_marker(mgpu_mark)