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 .
```

plugins/plugin/internal-functions

@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
 
 plugin_list_cmd() {
@@ -14,5 +15,5 @@ plugin_prime_bash_completion() {
   declare desc="primes the bash-completion cache"
 
   dokku_log_info1_quiet "Priming bash-completion cache"
-  dokku --quiet help --all | awk '/^    /{ print $1 }' | sort > "/var/cache/dokku-completion"
+  dokku --quiet help --all | awk '/^    /{ print $1 }' | sort >"/var/cache/dokku-completion"
 }