Fix test.json for boost test to match schema
Fix test.json for boost test to match schema

I guess the intent was that tests of thread/debug library variants only
get run with MSVC, but currently this test isn't getting run at all in
our Windows CI (since boost got removed from the VM image [1], and we
didn't notice, more on which anon).

[1] https://github.com/actions/virtual-environments/pull/2843

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