test: use constants for CSV/CLTV activation heights in rpc_signrawtransaction

test/functional/feature_cltv.py

@@ -9,6 +9,7 @@ Test that the CHECKLOCKTIMEVERIFY soft-fork activates at (regtest) block height
 """
 
 from test_framework.blocktools import (
+    CLTV_HEIGHT,
     create_block,
     create_coinbase,
 )
@@ -31,8 +32,6 @@ from test_framework.wallet import (
     MiniWalletMode,
 )
 
-CLTV_HEIGHT = 1351
-
 
 # Helper function to modify a transaction by
 # 1) prepending a given script to the scriptSig of vin 0 and