Allow DHM selftest to run if MBEDTLS_PEM_PARSE_C is unset
Allow DHM selftest to run if MBEDTLS_PEM_PARSE_C is unset

If MBEDTLS_PEM_PARSE_C is unset, the DHM selftest fails because
it uses PEM encoded test data.

This commit fixes this by providing the DER encoded form of the
test data instead in case MBEDTLS_PEM_PARSE_C is unset.

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