[tests] fix timeout issues from TestNode

Fixes a couple of bugs from the introduction of TestNode:

- test scripts were no longer able to specify a custom timeout for
starting a node. Therefore tests with nodes that take a long time to
start up (eg pruning.py) would fail.
- the test for whether a node has failed on start up was broken
by changing 'assert x is None' to 'assert not x'. Since
subprocess.poll() can return None (indicating the node is still running)
or 0 (indicating the node exited with return code 0), this was a
regression.

test/functional/dbcrash.py

@@ -64,7 +64,8 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         self.extra_args = [self.node0_args, self.node1_args, self.node2_args, self.node3_args]
 
     def setup_network(self):
-        self.setup_nodes()
+        # Need a bit of extra time for the nodes to start up for this test
+        self.nodes = self.start_nodes(self.num_nodes, self.options.tmpdir, self.extra_args, timewait=90)
         # Leave them unconnected, we'll use submitblock directly in this test
 
     def restart_node(self, node_index, expected_tip):
@@ -74,10 +75,10 @@ class ChainstateWriteCrashTest(BitcoinTestFramework):
         after 60 seconds. Returns the utxo hash of the given node."""
 
         time_start = time.time()
-        while time.time() - time_start < 60:
+        while time.time() - time_start < 120:
             try:
                 # Any of these RPC calls could throw due to node crash
-                self.nodes[node_index] = self.start_node(node_index, self.options.tmpdir, self.extra_args[node_index])
+                self.nodes[node_index] = self.start_node(node_index, self.options.tmpdir, self.extra_args[node_index], timewait=90)
                 self.nodes[node_index].waitforblock(expected_tip)
                 utxo_hash = self.nodes[node_index].gettxoutsetinfo()['hash_serialized_2']
                 return utxo_hash