Add test for ecdsa_read_signature_restartable()

Test values taken from a random signature as generated in
ecdsa_write_read_random() test function
diff --git a/include/mbedtls/ecdsa.h b/include/mbedtls/ecdsa.h
index f09e754..c7b9f27 100644
--- a/include/mbedtls/ecdsa.h
+++ b/include/mbedtls/ecdsa.h
@@ -62,7 +62,7 @@
  */
 typedef struct
 {
-    mbedtls_ecp_restart_ctx rs_ecp;     /*!<  base context (admin+ecp info) */
+    mbedtls_ecp_restart_ctx ecp;        /*!<  base context (admin+ecp info) */
 } mbedtls_ecdsa_restart_ctx;
 
 #else /* MBEDTLS_ECP_RESTARTABLE */