Change the logic of test_b008.c around the choice of the invalid skip index

Choose the invalid skip_index to use depending on how the test suite was
invoked: when running with relaxed version checking use the total number
of protocols advertised by the platform.
This will allow this test to be run successfully also against platform
implementing new, unknown protocols.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>