test/py: run sandbox in source directory

Some unit tests expect the cwd of the sandbox process to be the root
of the source tree. Ensure that requirement is met.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

test/py/u_boot_console_sandbox.py

@@ -44,7 +44,7 @@ class ConsoleSandbox(ConsoleBase):
             '-d',
             self.config.build_dir + '/arch/sandbox/dts/test.dtb'
         ]
-        return Spawn(cmd)
+        return Spawn(cmd, cwd=self.config.source_dir)
 
     def kill(self, sig):
         """Send a specific Unix signal to the sandbox process.