Add tests to verify std::function works with capturing lambda

Introduce a test that uses a capturing lambda as a custom_fake.

Signed-off-by: Yuval Peress <peress@google.com>

test/include/custom_function.hpp

@@ -0,0 +1,10 @@
+/* Copyright 2022 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#include <functional>
+
+#define CUSTOM_FFF_FUNCTION_TEMPLATE(RETURN, FUNCNAME, ...) \
+    std::function<RETURN (__VA_ARGS__)> FUNCNAME