test: check that combining PSBTs with different txs fails

[theStack]

This PR adds missing test coverage for the combinepsbt RPC, in the case of combining two PSBTs with different transactions:

bitcoin/src/psbt.cpp

Lines 24 to 27 in b8067cd

	// Prohibited to merge two PSBTs over different transactions
	if (tx->GetHash() != psbt.tx->GetHash()) {
	return false;
	}

The calling function CombinePSBTs checks for the false return value and then returns the transaction error string PSBT_MISMATCH:

bitcoin/src/psbt.cpp

Lines 433 to 435 in b8067cd

	if (!out.Merge(*it)) {
	return TransactionError::PSBT_MISMATCH;
	}

bitcoin/src/util/error.cpp

Lines 30 to 31 in b8067cd

	case TransactionError::PSBT_MISMATCH:
	return Untranslated("PSBTs not compatible (different transactions)");

[theStack]

Force-pushed a different variant of the first commit (extending the PSBT ctor), avoiding mutable objects as default parameters due to the following linter complaint:

A mutable list or dict seems to be used as default parameter value:

test/functional/test_framework/psbt.py:    def __init__(self, g=PSBTMap(), i=[], o=[]):

This is how mutable list and dict default parameter values behave:

>>> def f(i, j=[], k={}):
...     j.append(i)
...     k[i] = True
...     return j, k
...
>>> f(1)
([1], {1: True})
>>> f(1)
([1, 1], {1: True})
>>> f(2)
([1, 1, 2], {1: True, 2: True})

The intended behaviour was likely:

>>> def f(i, j=None, k=None):
...     if j is None:
...         j = []
...     if k is None:
...         k = {}
...     j.append(i)
...     k[i] = True
...     return j, k
...
>>> f(1)
([1], {1: True})
>>> f(1)
([1], {1: True})
>>> f(2)
([2], {2: True})

[instagibbs]

ACK b4f3322

[theStack]

Force-pushed with changes as suggested by @instagibbs (named arguments for PSBT ctor, adding call for success case of combining two identical PSBTs).

[instagibbs]

reACK 4e616d2

[achow101]

ACK 4e616d2