Test that starting bitcoind/bitcoin-qt with `-h` and `-version` works as expected.

Prompted by https://github.com/bitcoin/bitcoin/pull/10762#commitcomment-28345993, which is a nullpointer dereference triggered by starting bitcoin-qt with `-h`.

On master, this test passes when run over bitcoind, but fails when running over bitcoin-qt. I used xvfb as a virtual frame buffer to test:

```
BITCOIND=/home/ubuntu/bitcoin/src/qt/bitcoin-qt xvfb-run ./feature_help.py --nocleanup
2018-03-30T17:09:37.767000Z TestFramework (INFO): Initializing test directory /tmp/user/1000/testdi4dre13
2018-03-30T17:09:37.767000Z TestFramework (INFO): Start bitcoin with -h for help text
2018-03-30T17:09:37.841000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/home/ubuntu/bitcoin/test/functional/test_framework/test_framework.py", line 126, in main
    self.run_test()
  File "./feature_help.py", line 25, in run_test
    assert_equal(ret_code, 0)
  File "/home/ubuntu/bitcoin/test/functional/test_framework/util.py", line 39, in assert_equal
    raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
AssertionError: not(-11 == 0)
2018-03-30T17:09:37.842000Z TestFramework (INFO): Stopping nodes
Traceback (most recent call last):
  File "./feature_help.py", line 42, in <module>
    HelpTest().main()
  File "/home/ubuntu/bitcoin/test/functional/test_framework/test_framework.py", line 149, in main
    self.stop_nodes()
  File "/home/ubuntu/bitcoin/test/functional/test_framework/test_framework.py", line 273, in stop_nodes
    node.stop_node()
  File "/home/ubuntu/bitcoin/test/functional/test_framework/test_node.py", line 141, in stop_node
    self.stop()
  File "/home/ubuntu/bitcoin/test/functional/test_framework/test_node.py", line 87, in __getattr__
    assert self.rpc_connected and self.rpc is not None, "Error: no RPC connection"
AssertionError: Error: no RPC connection
```

Passes for bitcoind and bitcoin-qt when run on #12836.

Longer term, we should consider running functional tests over bitcoin-qt in one of the Travis jobs.

test_runner.py should also run the feature_help.py test and pass
test_runner.py feature_help should run and pass