Tests: fix TAP_AUTOTIME parsing in tap.c

The atoi() function can return '0' on error and is not guaranteed to set
errno accordingly. Use strtol() instead which can also return 0 on error
but will set errno properly.

Check errno after the function call to distinguish between the value '0'
and the error code '0'.

Change-Id: I5a82bb0f18d7e398dc3594aede5a38e6fc10dd7b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>