By having gossipwith filter out messages we don't want, we can get the counts of
expected messages correct, and not hit errors like this:
```
def test_gossip_throttle(node_factory, bitcoind, chainparams):
"""Make some gossip, test it gets throttled"""
l1, l2, l3, l4 = node_factory.line_graph(4, wait_for_announce=True,
opts=[{}, {}, {}, {'dev-throttle-gossip': None}])
# We expect: self-advertizement (3 messages for l1 and l4) plus
# 4 node announcements, 3 channel announcements and 6 channel updates.
# We also expect it to send a timestamp filter message.
# (We won't take long enough to get a ping!)
expected = 4 + 4 + 3 + 6 + 1
# l1 is unlimited
start_fast = time.time()
out1 = subprocess.run(['devtools/gossipwith',
'--all-gossip',
'--hex',
'--network={}'.format(TEST_NETWORK),
'--max-messages={}'.format(expected),
'{}@localhost:{}'.format(l1.info['id'], l1.port)],
check=True,
timeout=TIMEOUT, stdout=subprocess.PIPE).stdout.split()
time_fast = time.time() - start_fast
assert time_fast < 2
# Remove timestamp filter, since timestamp will change!
out1 = [m for m in out1 if not m.startswith(b'0109')]
# l4 is throttled
start_slow = time.time()
out2 = subprocess.run(['devtools/gossipwith',
'--all-gossip',
'--hex',
'--network={}'.format(TEST_NETWORK),
'--max-messages={}'.format(expected),
'{}@localhost:{}'.format(l4.info['id'], l4.port)],
check=True,
timeout=TIMEOUT, stdout=subprocess.PIPE).stdout.split()
time_slow = time.time() - start_slow
assert time_slow > 3
# Remove timestamp filter, since timestamp will change!
out2 = [m for m in out2 if not m.startswith(b'0109')]
# Contents should be identical (once uniquified, since each
# doubles-up on its own gossip)
> assert set(out1) == set(out2)
E AssertionError: assert {b'010054b1907bdf639c9060e0fa4bca02419c46f75a99f0908b87a2e09711d5d031ba76b8fd07acc8be1b2fac9e31efb808e5d362c32ef4665...
E Extra items in the left set:
E b'01010ad5be8b9ba029245c2ae2d667af7ead7c0129c479c7fd7145a9b65931e90222082e6e4ab37ef60ebd10f1493d73e8bf7a40c4ae5f7d87cc...8488830b60f7e744ed9235eb0b1ba93283b315c035180266e44a554e494f524245414d2d333930353033622d6d6f64646564000000000000000000'
E Extra items in the right set:
E b'01079f87eb580b9e5f11dc211e9fb66abb3699999044f8fe146801162393364286c6000000010000006c010101'
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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