cactus: do not compile irrelevant test files

Now Cactus image is only used for showcasing SPCI Beta1 spec features,
however there are some residual tests(related with SPRT) which are no
more relevant.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ia049f20f47c8d68d84b25a424d6a064592c3b0af