[PaulLodge]

A small fix so that tests in the core that don't need the suexec module can run without error whilst the single test class adds the module before running the test. That test will also be skipped if the mod_suexec is not present in the module/ directory.

---

• This PR does not report or address a security vulnerability
  (see SECURITY.md and
  https://www.apache.org/security/#reporting-a-vulnerability).
• Code follows the httpd style guide.
• New log messages (level debug or higher) use an empty APLOGNO() tag;
  numbers are assigned by a committer at merge time (see docs/log-message-tags/README).
• If the change is user-visible, a changes-entries/*.txt file is included,
  following the template in README.CHANGES (not needed otherwise).
• Test cases based on pyhttpd are included where appropriate, in the
  test/modules/xxx directory matching the modules/xxx module source,
  or test/modules/core for core server changes. On Unix, verify these
  pass locally with e.g. make check-pytest PYTEST_DIRS=test/modules/xxx.

[covener]

mod_headers is reasonable to leave in (notwithstanding that the CI failure might be showing that adding it back in didn't work?)