registry/storage/driver: checking that non-existent path returns PathNotFoundError

Issue #1186 describes a condition where a null tags response is returned when
using the s3 driver. The issue seems to be related to a missing
PathNotFoundError in s3. This change adds a test for that to get an idea of the
lack of compliance across storage drivers. If the failures are manageable,
we'll add this test condition and fix the s3 driver.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/storage/driver/testsuites/testsuites.go

@@ -472,6 +472,13 @@ func (suite *DriverSuite) TestList(c *check.C) {
 	rootDirectory := "/" + randomFilename(int64(8+rand.Intn(8)))
 	defer suite.StorageDriver.Delete(suite.ctx, rootDirectory)
 
+	doesnotexist := path.Join(rootDirectory, "nonexistent")
+	_, err := suite.StorageDriver.List(suite.ctx, doesnotexist)
+	c.Assert(err, check.Equals, storagedriver.PathNotFoundError{
+		Path:       doesnotexist,
+		DriverName: suite.StorageDriver.Name(),
+	})
+
 	parentDirectory := rootDirectory + "/" + randomFilename(int64(8+rand.Intn(8)))
 	childFiles := make([]string, 50)
 	for i := 0; i < len(childFiles); i++ {