test_dnf4_mark(): disable all plugins when inspecting markings

When subscription-manager DNF plugins are enabled (e.g. on RHEL), they
produce messages to the stdout on any DNF command execution. E.g.
"Updating Subscription Management repositories.".

Disable all plugins when inspecting package markings so prevent them
from modifying the output.

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

test/run/test_stages.py

@@ -541,6 +541,8 @@ class TestStages(test.TestBase):
                 [
                     "dnf",
                     "--installroot", tree,
+                    # disable all plugins to prevent them from modifying the output
+                    "--disableplugin", "*",
                     "repoquery", "--installed",
                     "--qf", "%{name},%{reason}\n"
                 ],