Test/assemblers/assertImageFile: don't set default argument values

Do not specify the default value for 'expected_size' argument in
assertImageFile() function declaration. Previously, it was set to
`None`, which was never taken into account. Moreover, all callers of the
function always provide an explicit value, so the default was never
really used.

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

test/run/test_assemblers.py

@@ -23,7 +23,7 @@ def osbuild_fixture():
     yield test.OSBuild()
 
 
-def assertImageFile(filename, fmt, expected_size=None):
+def assertImageFile(filename, fmt, expected_size):
     info = json.loads(subprocess.check_output(["qemu-img", "info", "--output", "json", filename]))
     assert info["format"] == fmt
     assert info["virtual-size"] == expected_size