Adds tiny rtc::StringFormat utility

Bug: webrtc:11493
Change-Id: If11a0362dfa820e4464129d0ea58ff8bc4ce86bc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173323
Commit-Queue: Henrik Andreassson <henrika@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31043}

rtc_base/strings/string_format_unittest.cc

@@ -0,0 +1,35 @@
+/*
+ *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "rtc_base/strings/string_format.h"
+
+#include "rtc_base/checks.h"
+#include "test/gtest.h"
+
+namespace rtc {
+
+TEST(StringFormatTest, Empty) {
+  EXPECT_EQ("", StringFormat("%s", ""));
+}
+
+TEST(StringFormatTest, Misc) {
+  EXPECT_EQ("123hello w", StringFormat("%3d%2s %1c", 123, "hello", 'w'));
+  EXPECT_EQ("3 = three", StringFormat("%d = %s", 1 + 2, "three"));
+}
+
+TEST(StringFormatTest, MaxSizeShouldWork) {
+  const int kSrcLen = 512;
+  char str[kSrcLen];
+  std::fill_n(str, kSrcLen, 'A');
+  str[kSrcLen - 1] = 0;
+  EXPECT_EQ(str, StringFormat("%s", str));
+}
+
+}  // namespace rtc