Regression test for pegin validation issues

Add a regression test for the preceding commits, checking that we can sync
successfully when making a bunch of new blocks just as we have transient
loss of parent daemon connectivity. This reliably fails without the fix, and
reliably succeeds with it. (It stands in for the situation, more common in
production, where we sync faster than the parent daemon can keep up after a
long outage.)

Author: gwillen

test/functional/feature_fedpeg.py

@@ -125,6 +125,7 @@ def setup_network(self, split=False):
                 '-parent_bech32_hrp=bcrt',
                 # Turn of consistency checks that can cause assert when parent node stops
                 # and a peg-in transaction fails this belt-and-suspenders check.
+                # NOTE: This can cause spurious problems in regtest, and should be dealt with in a better way.
                 '-checkmempool=0',
             ]
             if not self.options.parent_bitcoin:
@@ -470,9 +471,11 @@ def run_test(self):
         self.start_node(1)
         connect_nodes_bi(self.nodes, 0, 1)
 
-        # Don't make a block, race condition when pegin-invalid block
-        # is awaiting further validation, nodes reject subsequent blocks
-        # even ones they create
+        # Make some blocks right here, as a regression test for
+        # https://gl.blockstream.com/liquid/elements/-/issues/42 .
+        print("Generating some blocks, to stress-test handling of this scenario")
+        sidechain.generate(10)
+
         print("Now waiting for node to re-evaluate peg-in witness failed block... should take a few seconds")
         self.sync_all(self.node_groups)
         print("Completed!\n")