Add a helper method for setting the "reference time" of the self-updating repo

src/repositories/shuttle/eta/SelfUpdatingETARepository.ts

@@ -18,4 +18,11 @@ export interface SelfUpdatingETARepository extends ETAGetterRepository {
     identifier: ShuttleTravelTimeDataIdentifier,
     dateFilter: ShuttleTravelTimeDateFilterArguments
   ): Promise<number | undefined>;
+
+  /**
+   * Set the "current time" as the class knows it, in order to calculate
+   * ETAs based on past data.
+   * @param referenceTime
+   */
+  setReferenceTime(referenceTime: Date): void;
 }