Merge #16420: QA: Fix race condition in wallet_encryption test

024ecd7e01d436ad7de608e571a20c7284b3951b QA: Fix race condition in wallet_encryption test (Jonas Schnelli)

Pull request description:

  There is some imprecision probably in the internal HTTPRPCTimer class (haven't exactly figured out where).
  But we can't expect that waiting excatly 2 seconds right after calling `walletpassphrase(2)` will result in a locked wallet due to the nature how we internally handle threads/timers.

  The wallet_encryption test fails regularely in CIs.

  Here is a logged session:
  ```shell
   node0 2019-07-18T18:51:22.569739Z [] ThreadRPCServer method=walletpassphrase user=__cookie__ 
   node0 2019-07-18T18:51:22.628656Z [] queue run of timer lockwallet() in 2 seconds (using HTTP) 
   node0 2019-07-18T18:51:22.629002Z [] Received a POST request for / from 127.0.0.1:46898 
   node0 2019-07-18T18:51:22.629081Z [] ThreadRPCServer method=dumpprivkey user=__cookie__ 
   node0 2019-07-18T18:51:24.445620Z [] Flushing wallet.dat 
   node0 2019-07-18T18:51:24.451421Z [] Flushed wallet.dat 6ms 
   node0 2019-07-18T18:51:24.631703Z [] Received a POST request for / from 127.0.0.1:46898 
   node0 2019-07-18T18:51:24.631737Z [] ThreadRPCServer method=dumpprivkey user=__cookie__ 
   test  2019-07-18T18:51:24.632000Z TestFramework (ERROR): Assertion failed 
                                     Traceback (most recent call last):
                                       File "/home/ubuntu/src/test/functional/test_framework/test_framework.py", line 193, in main
                                         self.run_test()
                                       File "/home/ubuntu/src/test/functional/wallet_encryption.py", line 53, in run_test
                                         assert_raises_rpc_error(-13, "Please enter the wallet passphrase with walletpassphrase first", self.nodes[0].dumpprivkey, address)
  ```

ACKs for top commit:
  promag:
    ACK 024ecd7e01d436ad7de608e571a20c7284b3951b, simple fix, one second shouldn't hurt.
  MarcoFalke:
    ACK 024ecd7e01d436ad7de608e571a20c7284b3951b
  fanquake:
    ACK 024ecd7e01d436ad7de608e571a20c7284b3951b

Tree-SHA512: 0cda1b8969b084bb765d2b35e90a8611c565ee458a7be1f2dde675f8ddbd9b9e421514547a7683f836e2c996e0538eb66b8c5b935b5a81e9319fb2be27624374

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