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/test_deploy

@@ -1,7 +1,11 @@
 #!/usr/bin/env bash
 set -xeo pipefail
 
-SELF=$(which "$0"); APP="$1"; TARGET="$2"; FORWARDED_PORT="$3"; SHOULD_FAIL="$4"
+SELF=$(which "$0")
+APP="$1"
+TARGET="$2"
+FORWARDED_PORT="$3"
+SHOULD_FAIL="$4"
 REMOTE="dokku@$TARGET"
 REPO="test-$(basename "$APP")-$RANDOM"
 
@@ -25,7 +29,7 @@ succeeded() {
 TMP=$(mktemp -d "/tmp/$TARGET.XXXXX")
 rmdir "$TMP" && cp -r "$(dirname "$SELF")/$APP" "$TMP"
 
-pushd "$TMP" &> /dev/null || exit 1
+pushd "$TMP" &>/dev/null || exit 1
 trap 'popd &> /dev/null || true; rm -rf "$TMP"' INT TERM EXIT
 
 git init