The code tried to only write the RGB bytes of FORMAT_RGB24, however, the in-memory layout is different on big-endian which would result in the wrong bytes being written. Fix this by simply also writing the byte we do not care about.
Initially only the vfs5011 driver is tested. Please note that these tests will be unreliable before umockdev 0.13.2. See also https://github.com/martinpitt/umockdev/pull/92