test: after reindexing, only legitimate protected subjects are searchable

This test scans for all the possible protected headers (including
bogus/broken ones) that are present in the protected-headers corpus,
trying to make sure that only the ones that are not broken or
malformed show up in a search after re-indexing.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>