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/.websocket.disabled/check_deploy

@@ -1,2 +1,5 @@
 #!/usr/bin/env bash
-set -e; output="$(curl -s -S -i -N -H \"Connection: Upgrade\" -H \"Upgrade: websocket\" -H \"Host: $1\" -H \"Origin: $1\" $1 -m 2)"; echo $output; test "$output" == "bazingaa"
+set -e
+output="$(curl -s -S -i -N -H \"Connection: Upgrade\" -H \"Upgrade: websocket\" -H \"Host: $1\" -H \"Origin: $1\" $1 -m 2)"
+echo $output
+test "$output" == "bazingaa"