build-scripts/build-test-tf-a-tests.sh: Move TF_A_PLATFORM_VARIANT

TF_A_PLATFORM_VARIANT is currently parsed from TF-A build flags. This
breaks compatibility as older platforms do not have this flag defined.
Moving the storage of this flag to TFA_TESTS_TF_A_BUILD_FLAGS to be
defined directly.

Change-Id: I7fdd0fb6b0b38fc080271c6b0e8fd381b58491a3
Signed-off-by: Jerry Wang <Jerry.Wang4@arm.com>