test: introduce generate_to_height helper, use in rpc_signrawtransaction

This will speed up the test a bit and avoid potential .generate() RPC
timeouts (in sub-test `test_signing_with_cltv()`) on slower machines.

test/functional/test_framework/util.py

@@ -559,6 +559,17 @@ def mine_large_block(node, utxos=None):
     node.generate(1)
 
 
+def generate_to_height(node, target_height):
+    """Generates blocks until a given target block height has been reached.
+       To prevent timeouts, only up to 200 blocks are generated per RPC call.
+       Can be used to activate certain soft-forks (e.g. CSV, CLTV)."""
+    current_height = node.getblockcount()
+    while current_height < target_height:
+        nblocks = min(200, target_height - current_height)
+        current_height += len(node.generate(nblocks))
+    assert_equal(node.getblockcount(), target_height)
+
+
 def find_vout_for_address(node, txid, addr):
     """
     Locate the vout index of the given transaction sending to the