search: test indexed multi-branch search
Created by: keegancsmith
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We adapt the existing test for TestZoektSearchHEAD to instead test the new indexedSearchRequest abstraction. This will allow us to test the interaction between zoektIndexedRepos and zoektIndexedSearch.
The test code has had a few changes to it, but is the same functionality. Additionally we test the returned file URLs (an important component of what zoektSearch does).
We also remove the test for zoekt returning an error. Allowing this test makes the test cases harder to read, and doesn't provide that much value.
Child of https://github.com/sourcegraph/sourcegraph/pull/11952