bignum_tests: Refactored mpi_mod_raw_to/fromt_mont_rep
bignum_tests: Refactored mpi_mod_raw_to/fromt_mont_rep

This patch migrates the tests to use the `mbedtls_test_read_mpi_core()`.

Signed-off-by: Minos Galanakis <bb9c15d46ed7ecad5455c4c22ac024636068eda6@arm.com>

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls