phraser: move the fallbacks onto the deck (V-502)

internal/phraser/failure_test.go

@@ -13,16 +13,7 @@ import (
 // fallbacks_ru_v1.json break Go tests, which is the coupling this file removed.
 func isFallback(t *testing.T, key, sources, got string) bool {
 	t.Helper()
-	e, ok := DefaultFallbacks().file.Entries[key]
-	if !ok {
-		t.Fatalf("no fallback entry %q", key)
-	}
-	for _, v := range e.Variants {
-		if strings.ReplaceAll(v, "{sources}", sources) == got {
-			return true
-		}
-	}
-	return false
+	return DefaultFallbacks().deck().matches(key, map[string]string{"sources": sources}, got)
 }
 
 // A dead server must be distinguishable from bad phrasing. Both PhraseChat and