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/post-delete

@@ -1,16 +1,18 @@
 #!/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"
 
 app_post_delete() {
   declare desc="apps post-delete plugin trigger"
   local trigger="app_post_delete"
-  local APP="$1"; local IMAGE_REPO=$(get_app_image_repo "$APP")
+  local APP="$1"
+  local IMAGE_REPO=$(get_app_image_repo "$APP")
   if [[ -n $APP ]]; then
     # remove contents for apps that are symlinks to other folders
-    rm -rf "${DOKKU_ROOT:?}/$APP/" > /dev/null
+    rm -rf "${DOKKU_ROOT:?}/$APP/" >/dev/null
     # then remove the folder and/or the symlink
-    rm -rf "${DOKKU_ROOT:?}/$APP" > /dev/null
+    rm -rf "${DOKKU_ROOT:?}/$APP" >/dev/null
   fi
 
   # shellcheck disable=SC2046