test(history): assert a complete index only once the projection has settled

CI's Windows full suite saw the index answer State=ready with Indexed=Total=1
while a rescan was still pending, so SearchIndexProgress honestly reported
building and the test, which had only waited for the hit to appear, failed.
The test now retries until the projection reports every source indexed and
nothing pending, fails at once on any unreadable-source signal, and only then
asserts that no incomplete-index signal is raised.

Not reproduced locally: the base passed 20x on macOS, 90x under six concurrent
processes on Windows, and a full package run on Windows.

internal/frontend/serve/history_search_test.go

@@ -125,17 +125,31 @@ func TestHistorySearchSaysNothingExtraWhenTheIndexAnsweredForEverySource(t *test
 	waitHistorySearch(t, "zebra migration", 1)
 
 	for _, query := range []string{"zebra", "quux nonexistent"} {
-		rec := httptest.NewRecorder()
-		historySearch(rec, httptest.NewRequest(http.MethodGet, "/history/search?q="+url.QueryEscape(query), nil))
-		var got historySearchResponse
-		if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
-			t.Fatal(err)
-		}
-		if rec.Code != http.StatusOK {
-			t.Fatalf("%q: status %d body %q, want 200", query, rec.Code, rec.Body.String())
-		}
-		if got.Index.Partial || got.Index.Reason != "" || got.Index.Failed != 0 || got.Index.Indexed != got.Index.Total {
-			t.Fatalf("%q: a complete projection raised an incomplete-index signal: %+v", query, got.Index)
+		deadline := time.Now().Add(20 * time.Second)
+		for {
+			rec := httptest.NewRecorder()
+			historySearch(rec, httptest.NewRequest(http.MethodGet, "/history/search?q="+url.QueryEscape(query), nil))
+			var got historySearchResponse
+			if err := json.Unmarshal(rec.Body.Bytes(), &got); err != nil {
+				t.Fatal(err)
+			}
+			if rec.Code != http.StatusOK {
+				t.Fatalf("%q: status %d body %q, want 200", query, rec.Code, rec.Body.String())
+			}
+			if got.Index.Failed != 0 || got.Index.Reason == history.SearchIndexFailed {
+				t.Fatalf("%q: a readable source was reported as unreadable: %+v", query, got.Index)
+			}
+			settled := got.Index.State == "ready" && got.Index.Total > 0 && got.Index.Indexed == got.Index.Total
+			if settled && history.SearchIndexStatus().Pending == 0 {
+				if got.Index.Partial || got.Index.Reason != "" {
+					t.Fatalf("%q: a complete projection raised an incomplete-index signal: %+v", query, got.Index)
+				}
+				break
+			}
+			if time.Now().After(deadline) {
+				t.Fatalf("%q: the projection never settled: %+v", query, got.Index)
+			}
+			time.Sleep(10 * time.Millisecond)
 		}
 	}
 }