Site Health: Only get MySQL server information in tests where it's actually needed.

This resolves an issue with plugins like HyperDB, which doesn't have a database connection until a query is made.

Props xkon, Otto42, Clorith.
See #47606.
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git-svn-id: https://core.svn.wordpress.org/trunk@47015 1a063a9b-81f0-0310-95a4-ce76da25c4cd