Deflake index migration tests

/i/index/refresh uses the elasticsearch.url to refresh both indices
(default and additional.) I saw proxy failures for
/<additional-index-name>/_refresh calls in the logs. This commit fixes
these by allowing to refresh both indices with one URL as failing
refreshes are one possible source of flakiness.

Author: supersven

services/brig/test/integration/API/Search.hs

@@ -645,6 +645,9 @@ testMigrationToNewIndex opts brig migrateIndexCommand = do
                 & Opt.elasticsearchLens . Opt.urlLens .~ newESUrl
             optsWithIndex "both" =
               optsWithIndex "old"
+                -- Unfortunately, /i/index/refresh uses this URL to refresh both indices!
+                -- Thus, we need to refer to the proxy that hands requests to both through.
+                & Opt.elasticsearchLens . Opt.urlLens .~ bothIndexUrl
                 & Opt.elasticsearchLens . Opt.additionalWriteIndexLens ?~ newESIndex
                 & Opt.elasticsearchLens . Opt.additionalWriteIndexUrlLens ?~ newESUrl
                 -- 'additionalCaCertLens' needs to be added in order for brig to be able to reach both indices.