[CT] Add ability to download to/upload from swarming GS dir with numerical subdirs

ct/go/util/gs_test.go

 package util
 
 import (
         "fmt"
         "io/ioutil"
         "os"
         "path/filepath"
         "testing"
 
         "github.com/stretchr/testify/assert"
@@ skipping 117 matching lines @@
         }
         assert.False(t, result3)
 
         // Test with local timestamp missing.
         result4, err := gs.AreTimeStampsEqual(tmpDir+"dummy_name", "unit-tests/util/")
         if err == nil {
                 t.Error("Expected an error")
         }
         assert.False(t, result4)
 }
+
+// Will need a local valid google_storage_token.data file with read write access
+// to run the below test.
+func Auth_TestDownloadSwarmingArtifacts(t *testing.T) {
+        testPagesetsDirName := filepath.Join("unit-tests", "util", "page_sets")
+
+        gs, err := NewGsUtil(nil)
+        assert.Nil(t, err)
+
+        localDir := filepath.Join(os.TempDir(), "util_test")

[dogben, 2016/05/18 15:22:02]

nit: Always prefer ioutil.TempDir over os.TempDir() to allow running multiple
processes concurrently. E.g. ioutil.TempDir(nil, nil)

(I guess you might use os.TempDir() for a lock file or something that should
persist across runs but not across reboots, but it's hard to think of many
legitimate uses.)

[rmistry, 2016/05/19 11:54:11]

Done.

+        defer util.RemoveAll(localDir)
+        pageSetToIndex, err := gs.DownloadSwarmingArtifacts(localDir, testPagesetsDirName, "10k", 1, 2)
+        if err != nil {
+                t.Errorf("Unexpected error: %s", err)
+        }
+
+        // Examine contents of returned dictionary.
+        assert.Equal(t, 2, len(pageSetToIndex))
+        assert.Equal(t, 1, pageSetToIndex[filepath.Join(localDir, "1.py")])
+        assert.Equal(t, 2, pageSetToIndex[filepath.Join(localDir, "2.py")])
+        // Examine contents of the local directory.
+        files, err := ioutil.ReadDir(localDir)
+        assert.Nil(t, err)
+        assert.Equal(t, 2, len(files))
+        assert.Equal(t, "1.py", files[0].Name())
+        assert.Equal(t, "2.py", files[1].Name())
+}