pytest: Shorten traceback length by default

This produces a lot of code output which is not very helpful and is quite
annoying to wade through. Use the short format by default.

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

test/py/test.py

@@ -17,4 +17,9 @@ if __name__ == '__main__':
     # argv; py.test test_directory_name user-supplied-arguments
     args = [os.path.dirname(__file__) + '/tests']
     args.extend(sys.argv)
+
+    # Use short format by default
+    if not [arg for arg in args if '--tb=' in arg]:
+        args.append('--tb=short')
+
     sys.exit(pytest.main(args))