lib/test: use new internal helpers in library's code and tests

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/library/pkwrite.c b/library/pkwrite.c
index 1cb8374..3659cd9 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -64,12 +64,12 @@
 {
     mbedtls_ecp_group_id id = mbedtls_pk_get_group_id(pk);
 
-#if defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
+#if defined(MBEDTLS_HAVE_CURVE25519)
     if (id == MBEDTLS_ECP_DP_CURVE25519) {
         return 1;
     }
 #endif
-#if defined(MBEDTLS_ECP_DP_CURVE448_ENABLED)
+#if defined(MBEDTLS_HAVE_CURVE448)
     if (id == MBEDTLS_ECP_DP_CURVE448) {
         return 1;
     }