Added unit-tests for treeWalkPool (#1969)

* Added unit-tests for treeWalkPool
* Added unit tests for tree-walk-fs
* Added period at the end of all comments.
* FS/XL: Unified tree walk tests for both backends
* Added disk failure related tests for treewalk

Replaced removeRandomDisks with removeDiskN. There is no need to
randomize disks that fail while the distribution of chunks in XL during
erasure coding data is random.

test-utils_test.go

@@ -546,11 +546,13 @@ func removeRoots(roots []string) {
 	}
 }
 
-// removeRandomDisk - removes a count of random disks from a disk slice.
-func removeRandomDisk(disks []string, removeCount int) {
-	ints := randInts(len(disks))
-	for _, i := range ints[:removeCount] {
-		removeAll(disks[i-1])
+//removeDiskN - removes N disks from supplied disk slice.
+func removeDiskN(disks []string, n int) {
+	if n > len(disks) {
+		n = len(disks)
+	}
+	for _, disk := range disks[:n] {
+		removeAll(disk)
 	}
 }