use Cleanup(),TempDir() in test cases (#5723)

Replace defer with t.Cleanup().

Replace the combination of ioutil.TempDir, error checking
and defer os.RemoveAll() with Go testing.T's new TempDir()
helper.

Mark auxiliary functions as test helpers.

.github/workflows/docker.yml

@@ -16,7 +16,7 @@ jobs:
     steps:
       - uses: actions/setup-go@v2
         with:
-          go-version: "^1.15.4"
+          go-version: "1.15"
       - uses: actions/checkout@master
       - name: Prepare
         id: prep