sandbox: Enable time unit test command

Enable this command for sandbox.

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

configs/sandbox_defconfig

@@ -27,3 +27,4 @@ CONFIG_SOUND=y
 CONFIG_CMD_SOUND=y
 CONFIG_SOUND_SANDBOX=y
 CONFIG_DM_RTC=y
+CONFIG_CMD_UT_TIME=y