test: cmd: setexpr: add format string tests

Add test cases for the setexpr format string operator.

Signed-off-by: Roland Gaudig <roland.gaudig@weidmueller.com>
[trini: Enable on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 96c9f61..bcd82f7 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -71,6 +71,7 @@
 CONFIG_CMD_SPI=y
 CONFIG_CMD_USB=y
 CONFIG_CMD_AXI=y
+CONFIG_CMD_SETEXPR_FMT=y
 CONFIG_CMD_AB_SELECT=y
 CONFIG_BOOTP_DNS2=y
 CONFIG_CMD_PCAP=y