test: Fix --hostuser octal UID test flakiness

The test for issue #19800 depended on finding a system user with an
octal UID. This approach was fragile because system users found on the
host may have a different UID in the testing image.

Use the rootless user ID instead which is likely to be octal anyway as
new user IDs start with 1000.

Signed-off-by: Ricardo Branco <rbranco@suse.de>