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/tar/internal-functions

@@ -1,16 +1,17 @@
 #!/usr/bin/env bash
-set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
 
 tar_help_content_func() {
   declare desc="return tar plugin help content"
-  cat<<help_content
+  cat <<help_content
     tar:in <app>, Reads an tarball containing the app from stdin
     tar:from <app> <url>, Loads an app tarball from url
 help_content
 }
 
 tar_help_cmd() {
-  if [[ $1 = "tar:help" ]] ; then
+  if [[ $1 == "tar:help" ]]; then
     echo -e 'Usage: dokku tar[:COMMAND]'
     echo ''
     echo 'An alternative to using git, apps are loaded via tarballs instead.'
@@ -20,7 +21,7 @@ tar_help_cmd() {
   elif [[ $(ps -o command= $PPID) == *"--all"* ]]; then
     tar_help_content_func
   else
-    cat<<help_desc
+    cat <<help_desc
     tar, Deploy applications via tarball instead of git
 help_desc
   fi