contrib: Improvements to hardcoded seeds scripts

- Moved all seed related scripts to contrib/seeds for consistency
- Updated `makeseeds.py` to handle IPv6 and onions, fix regular
  expression for recent Bitcoin Core versions
- Fixed a bug in `generate-seeds.py` with regard to IPv6 parsing

contrib/seeds/nodes_test.txt

@@ -0,0 +1,11 @@
+# List of fixed seed nodes for testnet
+
+# Onion nodes
+thfsmmn2jbitcoin.onion
+it2pj4f7657g3rhi.onion
+nkf5e6b7pl4jfd4a.onion
+4zhkir2ofl7orfom.onion
+t6xj6wilh4ytvcs7.onion
+i6y6ivorwakd7nw3.onion
+ubqj4rsu3nqtxmtp.onion
+