test: Add some tests for setexpr

This command currently has no tests. Add some for basic assignment and the
integer operations.

Note that the default size for setexpr is ulong, which varies depending on
the build machine. So for sandbox on a 64-bit host, this means that the
default size is 64 bits.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/cmd/Makefile b/test/cmd/Makefile
index 859dcda..b2f71af 100644
--- a/test/cmd/Makefile
+++ b/test/cmd/Makefile
@@ -4,3 +4,4 @@
 
 obj-y += mem.o
 obj-$(CONFIG_CMD_MEM_SEARCH) += mem_search.o
+obj-y += setexpr.o