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/apps/pre-delete

@@ -1,11 +1,16 @@
 #!/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"
 
 apps_pre_delete() {
   declare desc="apps pre-delete plugin trigger"
   local trigger="apps_pre_delete"
-  local APP="$1"; local IMAGE_TAG="$2"; local IMAGE=$(get_deploying_app_image_name "$APP" "$IMAGE_TAG"); local CACHE_DIR="$DOKKU_ROOT/$APP/cache"; local CACHE_HOST_DIR="$DOKKU_HOST_ROOT/$APP/cache"
+  local APP="$1"
+  local IMAGE_TAG="$2"
+  local IMAGE=$(get_deploying_app_image_name "$APP" "$IMAGE_TAG")
+  local CACHE_DIR="$DOKKU_ROOT/$APP/cache"
+  local CACHE_HOST_DIR="$DOKKU_HOST_ROOT/$APP/cache"
   verify_app_name "$APP"
 
   if ! is_image_herokuish_based "$IMAGE"; then