tests: Add and include headers with function prototypes

Part 2 of fixing -Wmissing-prototypes warnings. This patch adds
headers with function prototypes and includes the headers into
source files. This fixes the warnings like this:

warning: no previous prototype for ‘function_name’
[-Wmissing-prototypes]

This patch is needed to sync compiler warning options between meson
and makefile.

TEST=running the following produces no warnings:
meson setup --wipe (to clean build directory)
ninja test

Change-Id: Ia1ff22deb2354569f277649c6575ef2d5ffbb6e0
Signed-off-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/63489
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>

tests/chip.c

@@ -29,6 +29,7 @@
 #include <stdio.h>
 #include <string.h>
 
+#include "tests.h"
 #include "chipdrivers.h"
 #include "flash.h"
 #include "io_mock.h"