tests: run mvdan/shfmt on test runs

While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.

The command used to reprocess everything is:

```shell
shfmt -l -bn -ci -i 2 -w .
```

tests/apps/multi/check_deploy

@@ -1,2 +1,3 @@
 #!/usr/bin/env bash
-set -e; curl -s -S "$1" | grep -q "Heroku Multi Buildpack on Dokku"
+set -e
+curl -s -S "$1" | grep -q "Heroku Multi Buildpack on Dokku"