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

ct/go/util/gs_test.go

Index: ct/go/util/gs_test.go
diff --git a/ct/go/util/gs_test.go b/ct/go/util/gs_test.go
index b78ac1fa412cabe1e318009dcdf661dd1d12b90f..943a2155fc21ab679db96434e28981dfaee9b8b5 100644
--- a/ct/go/util/gs_test.go
+++ b/ct/go/util/gs_test.go
@@ -135,3 +135,31 @@ func TestAreTimestampsEqual(t *testing.T) {
 	}
 	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, err := ioutil.TempDir("", "util_test_")
+	assert.Nil(t, err)
+	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())
+}