tests: create fresh harness for each docker dns test

Not actually sure this fixes the flaky tests, but seems
like it could be related.

drivers/docker/driver_unix_test.go

@@ -815,16 +815,12 @@ func Test_dnsConfig(t *testing.T) {
 	ci.Parallel(t)
 	testutil.DockerCompatible(t)
 
-	require := require.New(t)
-	harness := dockerDriverHarness(t, nil)
-	defer harness.Kill()
-
 	cases := []struct {
 		name string
 		cfg  *drivers.DNSConfig
 	}{
 		{
-			name: "nil DNSConfig",
+			name: "nil",
 		},
 		{
 			name: "basic",
@@ -843,6 +839,9 @@ func Test_dnsConfig(t *testing.T) {
 	}
 
 	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			harness := dockerDriverHarness(t, nil)
+
 			taskCfg := newTaskConfig("", []string{"/bin/sleep", "1000"})
 			task := &drivers.TaskConfig{
 				ID:        uuid.Generate(),
@@ -851,16 +850,20 @@ func Test_dnsConfig(t *testing.T) {
 				Resources: basicResources,
 				DNS:       c.cfg,
 			}
-		require.NoError(task.EncodeConcreteDriverConfig(&taskCfg))
+			require.NoError(t, task.EncodeConcreteDriverConfig(&taskCfg))
 
 			cleanup := harness.MkAllocDir(task, false)
-		defer cleanup()
 
 			_, _, err := harness.StartTask(task)
-		require.NoError(err)
-		defer harness.DestroyTask(task.ID, true)
+			require.NoError(t, err)
 
 			dtestutil.TestTaskDNSConfig(t, harness, task.ID, c.cfg)
+
+			// cleanup immediately before the next test case
+			require.NoError(t, harness.DestroyTask(task.ID, true))
+			cleanup()
+			harness.Kill()
+		})
 	}
 
 }