Add test for ecdsa_read_signature_restartable()

Test values taken from a random signature as generated in
ecdsa_write_read_random() test function

library/ecdsa.c

diff --git a/library/ecdsa.c b/library/ecdsa.c
index 3003812..19f9d5e 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -476,7 +476,7 @@
  */
 void mbedtls_ecdsa_restart_init( mbedtls_ecdsa_restart_ctx *ctx )
 {
-    mbedtls_ecp_restart_init( &ctx->rs_ecp );
+    mbedtls_ecp_restart_init( &ctx->ecp );
 }
 
 /*
@@ -484,7 +484,7 @@
  */
 void mbedtls_ecdsa_restart_free( mbedtls_ecdsa_restart_ctx *ctx )
 {
-    mbedtls_ecp_restart_free( &ctx->rs_ecp );
+    mbedtls_ecp_restart_free( &ctx->ecp );
 }
 #endif /* MBEDTLS_ECP_RESTARTABLE */