test runner: check if the service network already exists

While investigating issues locally it's annoying to remove the service network, because it blocks next tests from execution:

```
➜ zoom git:(pkg-changelogs-1) elastic-package test system --data-streams webhook -v
2021/02/17 10:57:05 DEBUG Enable verbose logging
Run system tests for the package
2021/02/17 10:57:05 DEBUG setting up service...
2021/02/17 10:57:05 DEBUG setting up service using Docker Compose service deployer
2021/02/17 10:57:05 DEBUG running command: /usr/local/bin/docker-compose -f /Users/marcin.tojek/go/src/github.com/elastic/integrations/packages/zoom/_dev/deploy/docker/docker-compose.yml -p elastic-package-service up --build -d
elastic-package-service_zoom-webhook-http_1 is up-to-date
Starting elastic-package-service_zoom-webhook-https_1 ... done
2021/02/17 10:57:06 DEBUG attaching service container elastic-package-service_zoom-webhook-https_1 to stack network elastic-package-stack_default
Error: error running package system tests: could not complete test run: could not setup service: could not attach service container to the stack network (stderr="Error response from daemon: endpoint with name elastic-package-service_zoom-webhook-https_1 already exists in network elastic-package-stack_default\n"): exit status 1
```

Start by reproducing the failure with `elastic-package test system --data-streams webhook -v` and inspect `_dev/deploy/docker/docker-compose.yml` plus the service-network setup invoked by the test runner. Done means rerunning system tests does not fail when the service container endpoint already exists in `elastic-package-stack_default`.

Written by the indexing model from the issue text.