Improve mbedtls_ecp_point_read_binary tests

Renamed the tests because they are explicitly testing Curve25519 and
nothing else. Improved test coverage, test documentation and extended
in-code documentation with a specific reference to the standard as well.

library/ecp.c

@@ -830,7 +830,7 @@ int mbedtls_ecp_point_read_binary( const mbedtls_ecp_group *grp,
         mbedtls_mpi_free( &pt->Y );
 
         if( grp->id == MBEDTLS_ECP_DP_CURVE25519 )
-            /* Set most significant bit to 0 */
+            /* Set most significant bit to 0 as prescribed in RFC7748 §5 */
             MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( &pt->X, plen * 8 - 1, 0 ) );
     }
 #endif