Tests for rustdoc search indices

[lifthrasiir]

The generation of search index has some delicate corner cases (e.g. #11391) that will benefit from tests. We at least need equivalence tests which compare a generated search index from given file with the expected one.

[steveklabnik]

Triage: no change.

[QuietMisdreavus]

Thanks to #47250, this exists now! Right now it's bound to the standard library's search index, but so far that's been good enough to test the problem cases we've had.