integration: Don't env cleanup before parallel subtests

Calling function returned from setupTest (which calls testEnv.Clean) in
a defer block inside a test that spawns parallel subtests caused the
cleanup function to be called before any of the subtest did anything.

Change the defer expressions to use `t.Cleanup` instead to call it only
after all subtests have also finished.
This only changes tests which have parallel subtests.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit f9e2eed55d)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

integration/plugin/common/plugin_test.go

@@ -32,7 +32,7 @@ import (
 // TestPluginInvalidJSON tests that POST endpoints that expect a body return
 // the correct error when sending invalid JSON requests.
 func TestPluginInvalidJSON(t *testing.T) {
-	defer setupTest(t)()
+	t.Cleanup(setupTest(t))
 
 	// POST endpoints that accept / expect a JSON body;
 	endpoints := []string{