Refactor autotest live network bootstrapping

- Use one node list and public bootstrap function for all autotests
- Use ifdefs for testnet/mainnet nodes
- Replace a few broken nodes with working ones

.github/scripts/flags-gcc.sh

@@ -17,3 +17,6 @@ add_flag -Wno-sign-compare
 add_flag -Wno-type-limits
 # Callbacks often don't use all their parameters.
 add_flag -Wno-unused-parameter
+# struct Foo foo = {0}; is a common idiom. Missing braces means we'd need to
+# write {{{0}}} in some cases, which is ugly and a maintenance burden.
+add_flag -Wno-missing-braces