[lib/cbuf] [app/tests] fix off by one in lib/cbuf. add a test.

Previously, if tail was == 0, and we wrote exactly enough bytes to
the end of the buffer, then head would end up at 0 as well. This
would make the buffer instaneously empty, as head == tail.

app/tests/rules.mk

@@ -5,6 +5,7 @@ MODULE := $(LOCAL_DIR)
 MODULE_SRCS += \
     $(LOCAL_DIR)/benchmarks.c \
     $(LOCAL_DIR)/cache_tests.c \
+    $(LOCAL_DIR)/cbuf_tests.c \
     $(LOCAL_DIR)/clock_tests.c \
     $(LOCAL_DIR)/fibo.c \
     $(LOCAL_DIR)/float.c \
@@ -18,6 +19,9 @@ MODULE_SRCS += \
 
 MODULE_ARM_OVERRIDE_SRCS := \
 
+MODULE_DEPS += \
+    lib/cbuf
+
 MODULE_COMPILEFLAGS += -Wno-format -fno-builtin
 
 include make/module.mk