test: Decode exceptions only with sandbox

When a real board fails we don't want to decode the exception. Reserve
that behaviour for sandbox. Also avoid raising a new exception on
failure - just re-raise the existing one.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/py/u_boot_console_sandbox.py

@@ -58,7 +58,7 @@ class ConsoleSandbox(ConsoleBase):
         if self.use_dtb:
             cmd += ['-d', self.config.dtb]
         cmd += self.sandbox_flags
-        return Spawn(cmd, cwd=self.config.source_dir)
+        return Spawn(cmd, cwd=self.config.source_dir, decode_signal=True)
 
     def restart_uboot_with_flags(self, flags, expect_reset=False, use_dtb=True):
         """Run U-Boot with the given command-line flags