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/scheduler-docker-local/post-delete

@@ -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/config/functions"
 
@@ -20,7 +21,7 @@ scheduler-docker-local-post-delete() {
   local DOKKU_APP_CIDS=$(docker ps -a --no-trunc | egrep "dokku/${APP}:" | awk '{ print $1 }' | xargs)
   if [[ -n "$DOKKU_APP_CIDS" ]]; then
     # shellcheck disable=SC2086
-    docker rm -f $DOKKU_APP_CIDS > /dev/null 2>&1 || true
+    docker rm -f $DOKKU_APP_CIDS >/dev/null 2>&1 || true
   fi
 
   # shellcheck disable=SC2046