Removed tests for directory mtimes.
Removed tests for directory mtimes.

In semantics.md we explicitly say we don't track any times but file
mtime.

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