test: scmi: add local variables for scmi agent reference

test: scmi: add local variables for scmi agent reference

Add local variables agent0/agent1 to refer to SCMI sandbox context
agent and ease readability of the test.

For consistency, rename regul_dev to regul0_dev and remove sandbox_voltd
in dm_test_scmi_voltage_domains().

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>