Tightened up tests for Nlink.
Tightened up tests for Nlink.

BanzaiMan/gcsfuse,GoogleCloudPlatform/gcsfuse,jacobsa/timeutil,jacobsa/ratelimit,BanzaiMan/gcsfuse,GoogleCloudPlatform/gcsfuse,mbrukman/gcsfuse,GoogleCloudPlatform/gcsfuse,kahing/gcsfuse