Several unit tests to improve test coverage of x509*.c

src/lib/crypt_ops/crypto_digest.c

src/lib/crypt_ops/crypto_digest.h

src/lib/tls/x509.c

src/lib/tls/x509_nss.c

src/lib/tls/x509_openssl.c

src/test/include.am

src/test/test.c

src/test/test.h

src/test/test_x509.c