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 .
```

tests/apps/python-flask/check_deploy

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
-set -e;
+set -e
 base_url=$1
-test_url () {
+test_url() {
   path="$1"
   expected_output="$2"
   url="${base_url}${path}"

tests/apps/python-flask/post-deploy

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-set -ex;
+set -ex
 
 REMOTE=$1
 REPO=$2