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>

contrib/pyln-testing/pyln/testing/utils.py

@@ -292,15 +292,43 @@ class TailableProc(object):
             except Exception:
                 pass
 
+    def readlines_wait_for_end(self, f, timeout=TIMEOUT):
+        """Read all complete lines from file object `f`.
+
+        If the last line is incomplete (no trailing newline), wait briefly
+        for it to complete before returning.
+
+        Returns list of lines including trailing newline.
+        """
+        lines = []
+        cur = ''
+        start = time.time()
+
+        while True:
+            line = f.readline()
+
+            if not line:
+                if cur != '':
+                    if time.time() - start > timeout:
+                        raise TimeoutError(f"Incomplete line never finished: {cur}")
+                    time.sleep(0.01)
+                    continue
+                return lines
+
+            cur += line
+            if cur.endswith('\n'):
+                lines.append(cur)
+                cur = ''
+
     def logs_catchup(self):
         """Save the latest stdout / stderr contents; return true if we got anything.
         """
-        new_stdout = self.stdout_read.readlines()
+        new_stdout = self.readlines_wait_for_end(self.stdout_read)
         if self.verbose:
             for line in new_stdout:
                 sys.stdout.write("{}: {}".format(self.prefix, line))
         self.logs += [l.rstrip() for l in new_stdout]
-        new_stderr = self.stderr_read.readlines()
+        new_stderr = self.readlines_wait_for_end(self.stderr_read)
         if self.verbose:
             for line in new_stderr:
                 sys.stderr.write("{}-stderr: {}".format(self.prefix, line))