membuf: Add some tests

Add tests for the membuf implementation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/lib/Makefile b/test/lib/Makefile
index 0e4cb8e..aa89923 100644
--- a/test/lib/Makefile
+++ b/test/lib/Makefile
@@ -14,6 +14,7 @@
 obj-$(CONFIG_SANDBOX) += kconfig.o
 obj-y += lmb.o
 obj-$(CONFIG_HAVE_SETJMP) += longjmp.o
+obj-$(CONFIG_SANDBOX) += membuf.o
 obj-$(CONFIG_CONSOLE_RECORD) += test_print.o
 obj-$(CONFIG_SSCANF) += sscanf.o
 obj-$(CONFIG_$(PHASE_)STRTO) += str.o