Fix cases where exponentiation was not fully tested
Fix cases where exponentiation was not fully tested

In two test cases, the exponentiation computation was not being fully tested
as when A_bytes (the base) == N_bytes (the modulus) -> A = N. When this is the
case A is reduced to 0 and therefore the result of the computation will always
be 0.

This fixes that issue and therefore increases the test coverage to ensure
different computations are actually being run.

Signed-off-by: Chris Jones <cbf609b812bf58762ceb89e8692559dbbe9149ea@arm.com>

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