Tightened up FakeGCS.ForeignModsTest.EmptySubDirectory.
Tightened up FakeGCS.ForeignModsTest.EmptySubDirectory.

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