Test/dnf4.mark: make failures to parse dnf output easier to debug

The test case still fails on RHEL-10.0 Beta, even when not using dnf5,
with:

```
            for line in r.stdout.splitlines():
>               package, mark = line.strip().split(",")
E               ValueError: not enough values to unpack (expected 2, got 1)
```

Make debugging of failures like this easier by printing the line when
the issue happens.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

test/run/test_stages.py

@@ -557,7 +557,12 @@ class TestStages(test.TestBase):
                 # Upstream bug https://github.com/rpm-software-management/dnf5/issues/709
                 if not line:
                     continue
-                package, mark = line.strip().split(",")
+
+                try:
+                    package, mark = line.strip().split(",")
+                except ValueError:
+                    print(f"Failed to parse line: {line}")
+                    raise
 
                 if package == "dnf":
                     assert mark == "user"