test(tc): return test failures count for tfm-testsuite

test(tc): return test failures count for tfm-testsuite

When running the "tfm-testsuite" set of platform tests, we now count
the number of failed tests (in addition to printing a test summary)
and report that back to the caller,
i.e. tc_bl31_common_platform_setup().

This will be useful to consolidate the tests failure reporting code in
a subsequent patch.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I8e51f03869f3b2f264b6581b3bd2a53be0198057