libbtrfsutil: python: lift the tests mkfs verbosity to a variable

The mkfs output is quite noisy in the test output so it's run with '-q',
however for debugging make it more visible in a variable.

[ci skip]

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

libbtrfsutil/python/tests/__init__.py

@@ -76,7 +76,8 @@ def mount_btrfs(self):
         else:
             mkfs = 'mkfs.btrfs'
         try:
-            subprocess.check_call([mkfs, '-q', image])
+            verbosity = '-q'
+            subprocess.check_call([mkfs, verbosity, image])
             subprocess.check_call(
                 [
                     'mount',