scmi_perf/test: Add further unit tests

Add unit tests for the following functions:
- scmi_perf_domain_attributes_handler
- scmi_perf_describe_levels_handler
- scmi_perf_describe_fast_channels
- find_opp_for_level
- validate_new_limits

Co-authored-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Signed-off-by: Chuyue Luo <Chuyue.Luo@arm.com>
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: Ie442634db79ff48b12a66a9335e6de2e51c4beaf