net: Fix sent reject messages for blocks and transactions

Ever since we #5913 have been sending invalid reject messages
for transactions and blocks.

test: Add basic test for `reject` code

Extend P2P test framework to make it possible to expect reject
codes for transactions and blocks.

Github-Pull: #7179
Rebased-From: 9fc6ed6003 20411903d7

qa/pull-tester/rpc-tests.py

@@ -100,6 +100,8 @@ testScripts = [
     'sendheaders.py',
     'keypool.py',
     'prioritise_transaction.py',
+    'invalidblockrequest.py',
+    'invalidtxrequest.py',
 ]
 testScriptsExt = [
     'bip65-cltv.py',
@@ -116,7 +118,6 @@ testScriptsExt = [
 #    'rpcbind_test.py', #temporary, bug in libevent, see #6655
     'smartfees.py',
     'maxblocksinflight.py',
-    'invalidblockrequest.py',
     'p2p-acceptblock.py',
     'mempool_packages.py',
     'maxuploadtarget.py',