gltesting: Ensure the Network enum matches the rust-bitcoin one

For a second I thought this might be the cause for a mixup between
`signet` and `regtest` but it turned out not to be the case. Anyway it
is likely less confusing for future debuggings if the numeric values
between these enums is the same, even if we don't really use the
rust-bitcoin enum with its numeric value.

libs/gl-testing/gltesting/utils.py

@@ -38,8 +38,12 @@ def get_node_version(self):
 
 
 class Network(Enum):
-    """Supported networks."""
+    """Supported networks.
+
+    Matches the enum in the `bitcoin::Network` rust code.
+    """
 
     BITCOIN = 0
     TESTNET = 1
-    REGTEST = 2
+    SIGNET = 2
+    REGTEST = 3