test: return container_id in make_container

The current `make_container()` helper is a bit silly (which is
entirely my fault). It requires a container tag as input but all
tests end up creating a random number for this input. So instead
just remove the input and return the container_id from the podman
build in the contextmanager and use that.

sources/test/test_container_storage_source.py

@@ -1,9 +1,7 @@
 #!/usr/bin/python3
 
 import os
-import random
 import socket
-import string
 import subprocess
 
 import pytest
@@ -16,10 +14,9 @@ SOURCES_NAME = "org.osbuild.containers-storage"
 @pytest.mark.skipif(not has_executable("podman"), reason="no podman executable")
 @pytest.mark.skipif(os.getuid() != 0, reason="root only")
 def test_containers_storage_integration(tmp_path, sources_module):
-    base_tag = "container-" + "".join(random.choices(string.digits, k=12))
-    with make_container(tmp_path, base_tag, {
+    with make_container(tmp_path, {
         "file1": "file1 content",
-    }):
+    }) as base_tag:
         image_id = subprocess.check_output(["podman", "inspect", "-f", "{{ .Id }}", base_tag],
                                            universal_newlines=True).strip()
         checksum = f"sha256:{image_id}"