sandbox: Enable env unit tests

Enable the new env unit tests on sandbox.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 2598017..e1d495c 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -40,3 +40,4 @@
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
+CONFIG_UT_ENV=y