pytest: fix flake involving partial lines.
Not sure how this happens, but it does, and may explain other races:
```
line = l1.daemon.wait_for_log(f"plugin-all_notifications.py: notification pay_failure: ")
dict_str = line.split("notification pay_failure: ", 1)[1]
> data = ast.literal_eval(dict_str)
tests/test_plugin.py:4869:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/ast.py:64: in literal_eval
node_or_string = parse(node_or_string.lstrip(" \t"), mode='eval')
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
source = "{'origin': 'pay', 'payload': {'payment_hash': 'a9caff840abedac582c3e22b46f78d205d8"
filename = '<unknown>', mode = 'eval'
def parse(source, filename='<unknown>', mode='exec', *,
type_comments=False, feature_version=None):
"""
Parse the source into an AST node.
Equivalent to compile(source, filename, mode, PyCF_ONLY_AST).
Pass type_comments=True to get back type comments where the syntax allows.
"""
flags = PyCF_ONLY_AST
if type_comments:
flags |= PyCF_TYPE_COMMENTS
if isinstance(feature_version, tuple):
major, minor = feature_version # Should be a 2-tuple.
assert major == 3
feature_version = minor
elif feature_version is None:
feature_version = -1
# Else it should be an int giving the minor version for 3.x.
> return compile(source, filename, mode, flags,
_feature_version=feature_version)
E File "<unknown>", line 1
E {'origin': 'pay', 'payload': {'payment_hash': 'a9caff840abedac582c3e22b46f78d205d8
E ^
E SyntaxError: unterminated string literal (detected at line 1)
/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/ast.py:50: SyntaxError
...
lightningd-1 2026-02-19T03:32:31.425Z INFO plugin-all_notifications.py: notification pay_failure: {'origin': 'pay', 'payload': {'payment_hash': 'a9caff840abedac582c3e22b46f78d205d87ff1212de05a64b12a6a53459bf29', 'bolt11': 'lnbcrt100n1p5edpz0sp5l5nj5p28kzm47d0tug2c2hz9q9gknd5sve6nwkf59fnpfvzecvcqpp54890lpq2hmdvtqkrug45daudypwc0lcjzt0qtfjtz2n22dzehu5sdq8v3jhxccxqyjw5qcqp9rzjqvuytqpdyk6wqaxvl47d3vee5swuwklej79qxjqqg394r4ptqaue5qqqvuqqqqgqqqqqqqqpqqqqqzsqqc9qxpqysgqvqzc5wd097xa6shdfkvr3yarvj36gu00ylfwdlfql9wkq3kueqgkk3pa3zdzd8a0de5vny0whzwjzrvhcuf86q6m9yakwjwjs3hzl9gpcf5lzn', 'error': {'message': 'failed: WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS (reply from remote)'}}, 'pay_failure': {'payment_hash': 'a9caff840abedac582c3e22b46f78d205d87ff1212de05a64b12a6a53459bf29', 'bolt11': 'lnbcrt100n1p5edpz0sp5l5nj5p28kzm47d0tug2c2hz9q9gknd5sve6nwkf59fnpfvzecvcqpp54890lpq2hmdvtqkrug45daudypwc0lcjzt0qtfjtz2n22dzehu5sdq8v3jhxccxqyjw5qcqp9rzjqvuytqpdyk6wqaxvl47d3vee5swuwklej79qxjqqg394r4ptqaue5qqqvuqqqqgqqqqqqqqpqqqqqzsqqc9qxpqysgqvqzc5wd097xa6shdfkvr3yarvj36gu00ylfwdlfql9wkq3kueqgkk3pa3zdzd8a0de5vny0whzwjzrvhcuf86q6m9yakwjwjs3hzl9gpcf5lzn', 'error': {'message': 'failed: WIRE_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS (reply from remote)'}}}
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
823a575d9a