test: avoid disk space warning for non-regtest

feature_config_args.py incorrectly assumed that its testnet4 node
would not log a disk space warning.

0683b8ebf3 increased m_assumed_blockchain_size
on testnet4 from 1 to 11 GiB which triggers this bug on more
systems, e.g. a RAM disk.

Prevent the warning by setting -prune for these nodes.

Fix the same issue in feature_signet.py

test/functional/feature_signet.py

@@ -26,12 +26,14 @@ signet_blocks = [
 
 class SignetParams:
     def __init__(self, challenge=None):
+        # Prune to prevent disk space warning on CI systems with limited space,
+        # when using networks other than regtest.
         if challenge is None:
             self.challenge = SIGNET_DEFAULT_CHALLENGE
-            self.shared_args = []
+            self.shared_args = ["-prune=550"]
         else:
             self.challenge = challenge
-            self.shared_args = [f"-signetchallenge={challenge}"]
+            self.shared_args = ["-prune=550", f"-signetchallenge={challenge}"]
 
 class SignetBasicTest(BitcoinTestFramework):
     def set_test_params(self):