tests: crypto: Update ECC Diffie-Hellman and DSA tests

These tests have been copied over from TinyCrypt and modified slightly
so that they compile on a non-POSIX system, with a minimal C library:
that's why it doesn't comply with Zephyr's coding style.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>