storage: tests: Ignore cases needing loop devices when not available

- In containers, loopback devices may not be available. Skip tests in this case
by looking at the output of losetup setup utility.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/storage/tests/test_storage.py

@@ -48,7 +48,13 @@ class Disk():
         command = 'losetup --show --find {file}'.format(
             file=self.disk_file.name)
         process = subprocess.run(command.split(), stdout=subprocess.PIPE,
-                                 check=True)
+                                 stderr=subprocess.PIPE)
+        if process.returncode:
+            if b'cannot find an unused loop device' in process.stderr:
+                pytest.skip('Loopback devices not available')
+            else:
+                raise Exception(process.stderr)
+
         device = process.stdout.decode().strip()
 
         subprocess.run(['partprobe', device], check=True)