ecp_curves: Added unit-tests for `secp256k1`

This patch introduces basic unit-testing for the `ecp_mod_p256k1()`.

The method is exposed through the ecp_invasive interface, and
the standard testing data is being provided by the python framework.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 1376f5d..21142b9 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -4617,7 +4617,8 @@
 int mbedtls_ecp_mod_p224k1(mbedtls_mpi *);
 #endif
 #if defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED)
-static int ecp_mod_p256k1(mbedtls_mpi *);
+MBEDTLS_STATIC_TESTABLE
+int ecp_mod_p256k1(mbedtls_mpi *);
 #endif
 
 #if defined(ECP_LOAD_GROUP)
@@ -5659,7 +5660,8 @@
  * Fast quasi-reduction modulo p256k1 = 2^256 - R,
  * with R = 2^32 + 2^9 + 2^8 + 2^7 + 2^6 + 2^4 + 1 = 0x01000003D1
  */
-static int ecp_mod_p256k1(mbedtls_mpi *N)
+MBEDTLS_STATIC_TESTABLE
+int ecp_mod_p256k1(mbedtls_mpi *N)
 {
     static mbedtls_mpi_uint Rp[] = {
         MBEDTLS_BYTES_TO_T_UINT_8(0xD1, 0x03, 0x00, 0x00, 0x01, 0x00, 0x00,