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/nodejs-worker/check_deploy

@@ -1,2 +1,5 @@
 #!/usr/bin/env bash
-set -e; output="$(pgrep -f "node worker.js" | wc -l)"; echo "$output"; test "$output" == "1"
+set -e
+output="$(pgrep -f "node worker.js" | wc -l)"
+echo "$output"
+test "$output" == "1"