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/checks/subcommands/enable

@@ -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/checks/functions"
 
@@ -7,7 +8,8 @@ checks_enable_cmd() {
   declare desc="enable zero-downtime checks for app/proctypes"
   local cmd="checks:enable"
   [[ "$1" == "$cmd" ]] && shift 1
-  local APP="$1"; verify_app_name "$APP"
+  local APP="$1"
+  verify_app_name "$APP"
   local PROCTYPES="${2:-_all_}"
 
   local DOKKU_CHECKS_DISABLED=$(config_get "$APP" DOKKU_CHECKS_DISABLED || true)