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>
diff --git a/test/dm/i2c.c b/test/dm/i2c.c
index 541b73b..c5939a1 100644
--- a/test/dm/i2c.c
+++ b/test/dm/i2c.c
@@ -66,6 +66,9 @@
 	uint8_t buf[5];
 
 	ut_assertok(uclass_get_device_by_seq(UCLASS_I2C, busnum, &bus));
+
+	/* Use test mode so we create the required errors for invalid speeds */
+	sandbox_i2c_set_test_mode(bus, true);
 	ut_assertok(i2c_get_chip(bus, chip, 1, &dev));
 	ut_assertok(dm_i2c_set_bus_speed(bus, 100000));
 	ut_assertok(dm_i2c_read(dev, 0, buf, 5));
@@ -73,6 +76,7 @@
 	ut_asserteq(400000, dm_i2c_get_bus_speed(bus));
 	ut_assertok(dm_i2c_read(dev, 0, buf, 5));
 	ut_asserteq(-EINVAL, dm_i2c_write(dev, 0, buf, 5));
+	sandbox_i2c_set_test_mode(bus, false);
 
 	return 0;
 }
@@ -100,7 +104,11 @@
 	struct udevice *bus, *dev;
 
 	ut_assertok(uclass_get_device_by_seq(UCLASS_I2C, busnum, &bus));
+
+	/* Use test mode so that this chip address will always probe */
+	sandbox_i2c_set_test_mode(bus, true);
 	ut_assertok(dm_i2c_probe(bus, SANDBOX_I2C_TEST_ADDR, 0, &dev));
+	sandbox_i2c_set_test_mode(bus, false);
 
 	return 0;
 }