test-driver.py: fix decoding of VM output

The codec format 'unicode_escape' was introduced in 52ee102 to handle
undecodable bytes in boot menus.

This made the problem worse as unicode chars outside of iso-8859-1
produce garbled output and valid utf-8 strings (such as "\x" ) trigger
decoding errors.

Fix this by using the default 'utf-8' codec and by explicitly ignoring
decoding errors.

Author: erikarvstedt

nixos/lib/test-driver/test-driver.py

@@ -704,7 +704,8 @@ def create_socket(path: str) -> socket.socket:
 
         def process_serial_output() -> None:
             for _line in self.process.stdout:
-                line = _line.decode("unicode_escape").replace("\r", "").rstrip()
+                # Ignore undecodable bytes that may occur in boot menus
+                line = _line.decode(errors="ignore").replace("\r", "").rstrip()
                 eprint("{} # {}".format(self.name, line))
                 self.logger.enqueue({"msg": line, "machine": self.name})