pytest: neaten test_cln_sendpay_weirdness, rename.

1. Allow 'any' as an option to zeroconf-selective.py plugin so we can use
   it in line_graph where we don't know ids yet.
2. Use modern helpers like line_graph and remove debugging statement.
3. Don't use listchannels(): it's true that it shows local channels as well,
   but that's a quirk I'd like to remove.
4. Make flake8 happy.
5. Rename to be more specific now it's a more narrow test.

I manually tested that the test still failed with the fixes removed, too,
so it is still the same test!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/zeroconf-selective.py

@@ -12,7 +12,7 @@ def on_openchannel(openchannel, plugin, **kwargs):
     plugin.log(repr(openchannel))
     mindepth = int(plugin.options['zeroconf-mindepth']['value'])
 
-    if openchannel['id'] == plugin.options['zeroconf-allow']['value']:
+    if openchannel['id'] == plugin.options['zeroconf-allow']['value'] or plugin.options['zeroconf-allow']['value'] == 'any':
         plugin.log(f"This peer is in the zeroconf allowlist, setting mindepth={mindepth}")
         return {'result': 'continue', 'mindepth': mindepth}
     else: