scmi_perf: Rearrange unit test directories

Rearrange unit test files under separate directories to divide
tests on different functionalities: main scmi-perf and the plugins
handler extension.

This will help to identify which tests belong to which source
file in the tree.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: Ib2fe8800cb642d3d1d707718c31ce970427bf3fe