sandbox: Allow selection of SPL unit tests

Now that we have more than one test, add a way to select the test to run.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/state.h b/arch/sandbox/include/asm/state.h
index f828d9d..7547602 100644
--- a/arch/sandbox/include/asm/state.h
+++ b/arch/sandbox/include/asm/state.h
@@ -93,6 +93,7 @@
 	bool show_of_platdata;		/* Show of-platdata in SPL */
 	bool ram_buf_read;		/* true if we read the RAM buffer */
 	bool run_unittests;		/* Run unit tests */
+	const char *select_unittests;	/* Unit test to run */
 
 	/* Pointer to information for each SPI bus/cs */
 	struct sandbox_spi_info spi[CONFIG_SANDBOX_SPI_MAX_BUS]