Fix ecp_check_pub() test cases
Fix ecp_check_pub() test cases

Negative x coordinate was tested with the value -1. It happens to be one
of the low order points both for Curve25519 and Curve448 and might be
rejected because of that and not because it is negative. Make sure that
x < 0 is the only plausible reason for the point to be rejected.

Signed-off-by: Janos Follath <b175c17abe0474719672be1367bb75ee28bd5c71@arm.com>

ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls