test/assemblers: do not fail on modprobe failure

This changes the `modprobe nbd` invocation to be non-fatal on failure,
since it might very well fail on reasonable setups. `modprobe` fails if
it cannot find a module in `/lib/modules`, even if it could reasonably
well figure out whether a module is already loaded. The reason is that
it needs the metadata from the module file to find the required modules
parameters.

If you have `nbd` already loaded but not mapped in `/lib/modules`, the
current call will cause test failures, even though the test would run
smoothly.

Fix this by never requiring `modprobe nbd` to succeed, but instead rely
on the tests failing if accessing `nbd` fails.

test/test_assemblers.py

@@ -14,7 +14,7 @@ class TestAssemblers(osbuildtest.TestCase):
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
-        subprocess.run(["modprobe", "nbd"], check=True)
+        subprocess.run(["modprobe", "nbd"], check=False)
 
     def run_assembler(self, name, options):
         with open("test/pipelines/f30-base.json") as f: