Previously `mbedtls_mpi_exp_mod` was tested with values that were over
`MBEDTLS_MPI_MAX_SIZE` in size. This is useful to do as some paths are only
taken when the exponent is large enough however, on builds where
`MBEDTLS_MPI_MAX_SIZE` is under the size of these test values.
This fix turns off these tests when `MBEDTLS_MPI_MAX_SIZE` is too small to
safely test (notably this is the case in config-thread.h).
Signed-off-by: Chris Jones <christopher.jones@arm.com>
4592bd8982