test: Make UT_LIB_ASN1 depend on sandbox

This doesn't seem to work on a real board, so use the test on sandbox
only.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/Kconfig b/test/Kconfig
index c3db727..310fb1b 100644
--- a/test/Kconfig
+++ b/test/Kconfig
@@ -32,6 +32,7 @@
 
 config UT_LIB_ASN1
 	bool "Unit test for asn1 compiler and decoder function"
+	depends on SANDBOX
 	default y
 	imply ASYMMETRIC_KEY_TYPE
 	imply ASYMMETRIC_PUBLIC_KEY_SUBTYPE