[#593] Fix testSmallDownloadAsync test

- Test was failling with:
generalError(message: "block not found in cache, should always be in cache in darkside mode")
- There was no wait so lightwalletd in darkside mode didn't have enough time to accept mocked state.

Tests/DarksideTests/BlockDownloaderTests.swift

@@ -31,6 +31,8 @@ class BlockDownloaderTests: XCTestCase {
         
         try FakeChainBuilder.buildChain(darksideWallet: darksideWalletService, branchID: branchID, chainName: chainName)
         try darksideWalletService.applyStaged(nextLatestHeight: 663250)
+
+        sleep(2)
     }
     
     override func tearDown() {