caddytest: close temp config file handle in validateTestPrerequisites

[vvdvortsova]

I'm not a native English speaker, so I used AI to help translate this PR description. The code and analysis are my own.

What

Fix resource leak in validateTestPrerequisites.
Fix for issue #7833.

validateTestPrerequisites in caddytest/caddytest.go created a temp file via os.CreateTemp and only removed it from disk in t.Cleanup (os.Remove(f.Name())), never closing the file descriptor including on the early return err if writing the config failed. Added defer f.Close() right after the file is created.

Testing
go build ./caddytest/...
go vet ./caddytest/...

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