Use `assetHelper.withCleaner` Within `ApiGwCliBasicTests.scala` Tests

[drcariel]

The tests within incubator-openwhisk-cli/tests/src/test/scala/whisk/core/cli/testApiGwCliBasicTests.scala tend to not use the assetHelper for several action/api tests. Instead, the action and/or api is cleaned up explicitly in each test's finally block via action.delete. This should be improved to use the assetHelper for more concise test code.

The other tests within this repo tend to use this assetHelper appropriately, but they all could use a look to make sure the usage is aligned.