ecp.py: Added tests for `mbedtls_ecp_mod_p255_raw`

This patch introduces the `EcpP255Raw` test class for testing
the curve using the preestablished `ecp_mod_p_generic_raw()`
test. The test's logic has been updated accordingly.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index 250efcc..cc8a327 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1349,6 +1349,13 @@
             curve_func = &mbedtls_ecp_mod_p256k1_raw;
             break;
 #endif
+#if defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED)
+        case MBEDTLS_ECP_DP_CURVE25519:
+            limbs = 2 * limbs_N;
+            curve_bits = 255;
+            curve_func = &mbedtls_ecp_mod_p255_raw;
+            break;
+#endif
         default:
             mbedtls_test_fail("Unsupported curve_id", __LINE__, __FILE__);
             goto exit;