The diagnostic report falsely reported chrome_binary as not found when
the config used ~ (e.g. ~/.chrome-for-testing/...). PathBuf::from()
treats ~ as a literal directory name, so existence checks always failed.
Add shellexpand::tilde() at the entry point of run_diagnostics() to
expand ~ before passing to downstream check functions. The original
unexpanded value is preserved for display in the report summary.
78f846e3c4