Reset errno to 0 after success

Reset errno to 0 after success

errno is only valid after a syscall wrapper (e.g. ioctl()) reports an
error. However, we report the last errno value as a part of the
igt_assert() output and so we display spurious values such as:

Test requirement not met in function gem_require_ring, file ioctl_wrappers.c:802:
Last errno: 11, Resource temporarily unavailable
Test requirement: (!((((intel_get_drm_devid(fd)) == 0x0102 || ...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>