test(tc): centralize platform error handling

test(tc): centralize platform error handling

Note that this change only affects the platform tests execution
path. It has no impact on the normal boot flow.

Make individual test functions propagate an error code, instead of
calling the platform error handler at the point of failure. The latter
is now the responsibility of the caller - in this case
tc_bl31_common_platform_setup().

Note that right now, tc_bl31_common_platform_setup() does not look at
the said error code but this initial change opens up an opportunity to
centralize any error handling in tc_bl31_common_platform_setup(),
which we will seize in subsequent patches.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: Ib282b64039e0b1ec6e6d29476fbaa2bcd33cb0c7