Ref T11589. This runs:

• preflight checks (critical checks: PHP version stuff, extensions);
• configuration;
• normal checks.

The PHP checks are split into critical ("bad version") and noncritical ("sub-optimal config").

I tidied up the extension checks slightly, we realistically depend on cURL nowadays.

• Faked a preflight failure.
• Hit preflight check.
• Got expected error screen.
• Loaded normal pages.
• Hit a normal setup check.
• Used DarkConsole "Startup" tab to verify that preflight checks take <1ms to run (we run them on every page without caching, at least for now, but they only do trivial checks like PHP versions).