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/src/fff_test_custom_function_template.cpp

@@ -0,0 +1,29 @@
+/* Copyright 2022 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <gtest/gtest.h>
+
+#include "fff_wrapper.hpp"
+
+DEFINE_FFF_GLOBALS
+
+FAKE_VOID_FUNC(do_stuff, int);
+
+class FFFCustomFakeSuite : public ::testing::Test {
+ public:
+  void SetUp() override {
+    RESET_FAKE(do_stuff);
+    FFF_RESET_HISTORY();
+  }
+};
+
+TEST_F(FFFCustomFakeSuite, custom_cpp_fake_function)
+{
+  int x = 0;
+  do_stuff_fake.custom_fake = [&x](int i) {
+    x = i;
+  };
+  do_stuff(5);
+  ASSERT_EQ(5, x);
+}