test: sandbox: add test for erase command

Add test for the erase command tested on ENV in EXT4.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index e91e9de..78d732d 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -24,6 +24,7 @@
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y
+CONFIG_CMD_ERASEENV=y
 CONFIG_CMD_NVEDIT_INFO=y
 CONFIG_CMD_NVEDIT_LOAD=y
 CONFIG_CMD_NVEDIT_SELECT=y