Implemented missing implementation files

I missed some implementation when I was looking over them. These are
them.

internal/platform/implementation/linux/timer_test.cc

@@ -0,0 +1,68 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "internal/platform/implementation/timer.h"
+
+#include <chrono>  // NOLINT
+// NOLINT
+#include <memory>
+#include <thread>  // NOLINT
+
+#include "gtest/gtest.h"
+#include "internal/platform/implementation/platform.h"
+
+namespace nearby {
+namespace linux {
+namespace {
+
+TEST(Timer, TestCreateTimer) {
+  int count = 0;
+
+  std::unique_ptr<nearby::api::Timer> timer =
+      nearby::api::ImplementationPlatform::CreateTimer();
+
+  ASSERT_TRUE(timer != nullptr);
+  EXPECT_FALSE(timer->Create(-100, 0, [&]() { ++count; }));
+  EXPECT_TRUE(timer->Stop());
+}
+
+// This test case cannot run on Google3
+TEST(Timer, DISABLED_TestRepeatTimer) {
+  int count = 0;
+
+  std::unique_ptr<nearby::api::Timer> timer =
+      nearby::api::ImplementationPlatform::CreateTimer();
+
+  ASSERT_TRUE(timer != nullptr);
+  EXPECT_TRUE(timer->Create(300, 300, [&]() { ++count; }));
+  std::this_thread::sleep_for(std::chrono::seconds(1));
+  EXPECT_TRUE(timer->Stop());
+  EXPECT_EQ(count, 3);
+}
+
+TEST(Timer, DISABLED_TestFireNow) {
+  int count = 0;
+
+  auto timer = nearby::api::ImplementationPlatform::CreateTimer();
+
+  EXPECT_TRUE(timer != nullptr);
+  EXPECT_TRUE(timer->Create(3000, 3000, [&]() { ++count; }));
+  EXPECT_TRUE(timer->FireNow());
+  EXPECT_TRUE(timer->Stop());
+  EXPECT_EQ(count, 1);
+}
+
+}  // namespace
+}  // namespace linux
+}  // namespace nearby