Add ecp_check_prvkey, with test

Also group key checking and generation functions in ecp.h and ecp.c.
diff --git a/tests/suites/test_suite_ecp.data b/tests/suites/test_suite_ecp.data
index 30fc461..d19e418 100644
--- a/tests/suites/test_suite_ecp.data
+++ b/tests/suites/test_suite_ecp.data
@@ -245,6 +245,9 @@
 depends_on:POLARSSL_ECP_DP_SECP521R1_ENABLED
 ecp_tls_write_read_group:SECP521R1
 
+ECP check prvkey
+ecp_check_prvkey:SECP192R1
+
 ECP gen keypair
 depends_on:POLARSSL_ECP_DP_SECP192R1_ENABLED
 ecp_gen_keypair:SECP192R1