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/docker-options/subcommands/default

@@ -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"
 source "$PLUGIN_AVAILABLE_PATH/docker-options/functions"
 
@@ -9,7 +10,7 @@ docker_options_main_cmd() {
   dokku_log_warn "Deprecated: Please use docker-options:report"
 
   verify_app_name "$2" && local APP="$2"
-  read -ra passed_phases <<< "$(get_phases "$3")"
+  read -ra passed_phases <<<"$(get_phases "$3")"
   if [[ ! "${passed_phases[@]}" ]]; then
     display_all_phases_options
   else