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

0683b8e 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

Github-Pull: bitcoin#32057
Rebased-From: 20fe41e

Github-Pull: bitcoin#32122
Rebased-From: fa5674c

bitcoin#30746 (comment) introduced an unsequenced operations with side-effects - which is undefined behavior, i.e. the right hand side can be evaluated before the left hand side, which happens to mutate it.

Tried:
```
clang++ --analyze -std=c++20 -I./src -I./src/test -I./src/test/fuzz src/test/fuzz/base_encode_decode.cpp src/psbt.cpp
```
but it didn't warn about UB.

Grepped for similar ones, but could find any other one in the codebase:
> grep -rnE --include='*.cpp' --include='*.h' '\b(\w+)\(([^)]*\b(\w+)\b[^)]*)\)\s*==\s*\3\.' .
```
./src/test/arith_uint256_tests.cpp:373:    BOOST_CHECK(R1L.GetHex() == R1L.ToString());
./src/test/arith_uint256_tests.cpp:374:    BOOST_CHECK(R2L.GetHex() == R2L.ToString());
./src/test/arith_uint256_tests.cpp:375:    BOOST_CHECK(OneL.GetHex() == OneL.ToString());
./src/test/arith_uint256_tests.cpp:376:    BOOST_CHECK(MaxL.GetHex() == MaxL.ToString());
./src/test/fuzz/cluster_linearize.cpp:565:        assert(depgraph.FeeRate(best_anc.transactions) == best_anc.feerate);
./src/test/fuzz/cluster_linearize.cpp:646:        assert(depgraph.FeeRate(found.transactions) == found.feerate);
./src/test/fuzz/cluster_linearize.cpp:765:            assert(depgraph.FeeRate(chunk_info.transactions) == chunk_info.feerate);
./src/test/fuzz/base_encode_decode.cpp:95:    assert(DecodeBase64PSBT(psbt, random_string, error) == error.empty());
./src/test/fuzz/key.cpp:102:        assert(pubkey.data() == pubkey.begin());
./src/test/skiplist_tests.cpp:42:        BOOST_CHECK(vIndex[from].GetAncestor(0) == vIndex.data());
./src/script/signingprovider.cpp:535:                   ComputeTapbranchHash(node.sub[1]->hash, node.sub[1]->hash) == node.hash) {
./src/pubkey.h:78:      return vch.size() > 0 && GetLen(vch[0]) == vch.size();
./src/cluster_linearize.h:881:            Assume(elem.inc.feerate.IsEmpty() == elem.pot_feerate.IsEmpty());
```

Hodlinator deduced the UB on Windows in bitcoin#32135 (comment)

Github-Pull: bitcoin#32141
Rebased-From: b1de59e

Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>