Properly set `$subdomain_error` to `false` when applicable in `ms_subdomain_constants()`. This was previously untestable because it used static vars: use globals instead.

Adds unit test.
Fixes #28697.

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git-svn-id: http://core.svn.wordpress.org/trunk@28984 1a063a9b-81f0-0310-95a4-ce76da25c4cd