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.

test/mod/test_testutil_make_container.py

@@ -0,0 +1,29 @@
+#
+# Tests for the 'osbuild.util.testutil.make_container'.
+#
+import subprocess
+import textwrap
+
+import pytest
+
+from osbuild.testutil import has_executable, make_container, mock_command
+
+
+def test_make_container_bad_podman_prints_podman_output(tmp_path, capsys):
+    fake_broken_podman = textwrap.dedent("""\
+    #!/bin/sh
+    echo fake-broken-podman
+    exit 1
+    """)
+    with mock_command("podman", fake_broken_podman):
+        with pytest.raises(subprocess.CalledProcessError):
+            with make_container(tmp_path, {}) as _:
+                pass
+    assert "fake-broken-podman" in capsys.readouterr().out
+
+
+@pytest.mark.skipif(not has_executable("podman"), reason="no podman executable")
+def test_make_container_integration(tmp_path, capsys):
+    with make_container(tmp_path, {"/etc/foo": "foo-content"}) as cref:
+        assert len(cref) == 64
+    assert "COMMIT" in capsys.readouterr().out