test/py: use " for docstrings

Python's coding style docs indicate to use " not ' for docstrings.

test/py has other violations of the coding style docs, since the docs
specify a stranger style than I would expect, but nobody has complained
about those yet:-)

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_sandbox_exit.py b/test/py/tests/test_sandbox_exit.py
index 1ec3607..d1aa308 100644
--- a/test/py/tests/test_sandbox_exit.py
+++ b/test/py/tests/test_sandbox_exit.py
@@ -9,14 +9,14 @@
 @pytest.mark.boardspec('sandbox')
 @pytest.mark.buildconfigspec('reset')
 def test_reset(u_boot_console):
-    '''Test that the "reset" command exits sandbox process.'''
+    """Test that the "reset" command exits sandbox process."""
 
     u_boot_console.run_command('reset', wait_for_prompt=False)
     assert(u_boot_console.validate_exited())
 
 @pytest.mark.boardspec('sandbox')
 def test_ctrl_c(u_boot_console):
-    '''Test that sending SIGINT to sandbox causes it to exit.'''
+    """Test that sending SIGINT to sandbox causes it to exit."""
 
     u_boot_console.kill(signal.SIGINT)
     assert(u_boot_console.validate_exited())