test: Fix debug_log_size helper

debug_log_bytes returned "an opaque number when in text mode"
(https://docs.python.org/3/tutorial/inputoutput.html#methods-of-file-objects),
not the number of bytes.

Fix this by using binary mode or text mode (with the same encoding)
consistently when opening the file for ftell() and read().

test/functional/test_framework/test_node.py

@@ -424,8 +424,8 @@ class TestNode():
     def wallets_path(self) -> Path:
         return self.chain_path / "wallets"
 
-    def debug_log_bytes(self) -> int:
-        with open(self.debug_log_path, encoding='utf-8') as dl:
+    def debug_log_size(self, **kwargs) -> int:
+        with open(self.debug_log_path, **kwargs) as dl:
             dl.seek(0, 2)
             return dl.tell()
 
@@ -434,7 +434,7 @@ class TestNode():
         if unexpected_msgs is None:
             unexpected_msgs = []
         time_end = time.time() + timeout * self.timeout_factor
-        prev_size = self.debug_log_bytes()
+        prev_size = self.debug_log_size(encoding="utf-8")  # Must use same encoding that is used to read() below
 
         yield
 
@@ -465,7 +465,7 @@ class TestNode():
             the number of log lines we encountered when matching
         """
         time_end = time.time() + timeout * self.timeout_factor
-        prev_size = self.debug_log_bytes()
+        prev_size = self.debug_log_size(mode="rb")  # Must use same mode that is used to read() below
 
         yield