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.

test/py/u_boot_spawn.py

@@ -20,11 +20,13 @@ class Spawn(object):
     sent to the process, and responses waited for.
     """
 
-    def __init__(self, args):
+    def __init__(self, args, cwd=None):
         """Spawn (fork/exec) the sub-process.
 
         Args:
-            args: array of processs arguments. argv[0] is the command to execute.
+            args: array of processs arguments. argv[0] is the command to
+              execute.
+            cwd: the directory to run the process in, or None for no change.
 
         Returns:
             Nothing.
@@ -44,6 +46,8 @@ class Spawn(object):
                 # run under "go" (www.go.cd). Perhaps this happens under any
                 # background (non-interactive) system?
                 signal.signal(signal.SIGHUP, signal.SIG_DFL)
+                if cwd:
+                    os.chdir(cwd)
                 os.execvp(args[0], args)
             except:
                 print 'CHILD EXECEPTION:'