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/pre-deploy

@@ -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 "$PLUGIN_CORE_AVAILABLE_PATH/common/property-functions"
 source "$PLUGIN_AVAILABLE_PATH/config/functions"
@@ -27,7 +28,7 @@ scheduler-docker-local-pre-deploy() {
     CONTAINER_PATHS=$(echo "$APP_PATHS" | awk -F ':' '{ print $2 }' | xargs)
     DOCKER_ARGS=$(: | plugn trigger docker-args-deploy "$APP" "$IMAGE_TAG")
     # strip --restart args from DOCKER_ARGS
-    DOCKER_ARGS=$(sed -e "s/--restart=[[:graph:]]\+[[:blank:]]\?//g" <<< "$DOCKER_ARGS")
+    DOCKER_ARGS=$(sed -e "s/--restart=[[:graph:]]\+[[:blank:]]\?//g" <<<"$DOCKER_ARGS")
     eval "ARG_ARRAY=($DOCKER_ARGS)"
   fi