sandbox: Allow SPI flash bootdevs to be disabled for tests

Most tests don't want these and they can create a lot of noise. Add a way
to disable them. Use that in tests, with a flag provided to enable them
for tests that need this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/test-main.c b/test/test-main.c
index 9d0f664..ea959f4 100644
--- a/test/test-main.c
+++ b/test/test-main.c
@@ -309,6 +309,7 @@
 		 * only set this if we know the ethernet uclass will be created
 		 */
 		eth_set_enable_bootdevs(test->flags & UT_TESTF_ETH_BOOTDEV);
+		test_sf_set_enable_bootdevs(test->flags & UT_TESTF_SF_BOOTDEV);
 		ut_assertok(dm_extended_scan(false));
 	}