testing: various cleanups [(#3877)](https://github.com/GoogleCloudPlatform/python-docs-samples/issues/3877)
testing: various cleanups [(#3877)](https://github.com/GoogleCloudPlatform/python-docs-samples/issues/3877)

* testing: various cleanups

* [iap]: only run iap tests on Kokoro
* [vision/automl]: use temporary directory for temporary files
* [appengine/flexible/scipy]: use temporary directory
* [bigtable/snippets/reads]: update pytest snapshot
* [texttospeech/cloud-client]: added output.mp3 to .gitignore
* [iot/api-client/gcs_file_to_device]: use temporary directory
* [iot/api-client/mqtt_example]: use temporary directory
* [logging/cloud-client]: use uuid and add backoff

* use project directory with Trampoline V1
googleapis/python-logging,googleapis/python-logging