Use mbedtls_test_read_mpi_core() to read the test data. Among other
benefits, X and Y are now allocated to their exact size, so analyzers (Asan,
Valgrind, Coverity, ...) have a chance of complaining if the tested function
overflows the buffer.
Remove TEST_CF_PUBLIC calls which are no longer necessary.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>