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/python-flask/post-deploy

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-set -ex;
+set -ex
 
 REMOTE=$1
 REPO=$2