This is an issue tracking all requested refactors of Cloud Functions PHP samples.

• Add start scripts to composer.json
• Use trait-based request method instead of raw POSTs in integration tests (e.g. $this->client->request('POST' --> $this->request(CloudEvent::fromArray($cloudevent));)
• Use processFunctionLogs helper.
• New: Replace assertContains() with assertStringContainsString()