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 .
```

debian/postrm

@@ -1,5 +1,6 @@
 #!/bin/bash
-set -eo pipefail; [[ $TRACE ]] && set -x
+set -eo pipefail
+[[ $TRACE ]] && set -x
 
 if [[ -e /usr/share/debconf/confmodule ]]; then
   # shellcheck disable=SC1091