Add dm unit tests that drive every pmbus subcommand handler against the sandbox PMBus chip emulator: dev (select by bus:addr and by regulator-name), list, info, telemetry, status, dump, read, write, clear, vout, scan and help. Tested using: ./u-boot -T -c "ut dm pmbus*" Signed-off-by: Vincent Jardin <vjardin@free.fr> Reviewed-by: Simon Glass <sjg@chromium.org> Signed-off-by: Peng Fan <peng.fan@nxp.com>