dm: i2c: Add an explicit test mode to the sandbox I2C driver

At present this driver has a few test features. They are needed for running
the driver model unit tests but are confusing and unnecessary if using
sandbox at the command line. Add a flag to enable the test mode, and don't
enable it by default.

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

include/i2c.h

@@ -54,6 +54,7 @@ struct dm_i2c_chip {
 	uint flags;
 #ifdef CONFIG_SANDBOX
 	struct udevice *emul;
+	bool test_mode;
 #endif
 };