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/proxy/subcommands/set

@@ -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"
 
@@ -8,7 +9,8 @@ proxy_set_cmd() {
   local cmd="proxy:set"
   [[ -z $2 ]] && dokku_log_fail "Please specify an app to run the command on"
 
-  local APP="$2"; verify_app_name "$APP"
+  local APP="$2"
+  verify_app_name "$APP"
   local PROXY_TYPE="$3"
 
   if [[ -n "$PROXY_TYPE" ]]; then