test/system: Fix the shebang

The setup_suite.bash file is meant to be written in Bash, and is not
supposed to have any Bats-specific syntax.  That's why it has the *.bash
suffix, not *.bats.  If Bats finds a setup_suite.bash file, when running
the test suite, it uses Bash's source(1) builtin to read the file.

This is a cosmetic change.  The Bats syntax is a superset of the Bash
syntax.  Therefore, it didn't make a difference to external tools like
Coverity that use the shebang to deduce the shell for shellcheck(1).
Secondly setup_suite.bash isn't meant to be an executable script and,
hence, the shebang has no effect on how the file is used.  However, it's
still a commonly used hint about the contents of the file, and it's
better to be accurate than misleading.

A subsequent commit will replace the shebangs in the test suite with
ShellCheck's 'shell' directives.

Fallout from 7a387dcc8b

https://github.com/containers/toolbox/pull/1363

test/system/setup_suite.bash

@@ -1,4 +1,4 @@
-#!/usr/bin/env bats
+#!/usr/bin/env bash
 #
 # Copyright © 2021 – 2023 Red Hat, Inc.
 #