Tests: Add InodeWatcher and FileWatcher tests

This patch adds some rudimentary tests for InodeWatcher.  It tests the
basic functionality, but maybe there are corner cases I haven't caught.
Additionally, this is our first LibCore test. :^)

Tests/LibCore/CMakeLists.txt

@@ -0,0 +1,8 @@
+set(
+  TEST_SOURCES
+  ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCoreFileWatcher.cpp
+)
+
+foreach(source ${TEST_SOURCES})
+  serenity_test(${source} LibCore)
+endforeach()

Tests/LibCore/TestLibCoreFileWatcher.cpp

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/API/InodeWatcherEvent.h>
+#include <LibCore/EventLoop.h>
+#include <LibCore/FileWatcher.h>
+#include <LibCore/Timer.h>
+#include <LibTest/TestCase.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+TEST_CASE(file_watcher_child_events)
+{
+    auto event_loop = Core::EventLoop();
+    auto maybe_file_watcher = Core::FileWatcher::create();
+    EXPECT_NE(maybe_file_watcher.is_error(), true);
+
+    auto file_watcher = maybe_file_watcher.release_value();
+    auto watch_result = file_watcher->add_watch("/tmp/",
+        Core::FileWatcherEvent::Type::ChildCreated
+            | Core::FileWatcherEvent::Type::ChildDeleted);
+    EXPECT_NE(watch_result.is_error(), true);
+
+    int event_count = 0;
+    file_watcher->on_change = [&](Core::FileWatcherEvent const& event) {
+        if (event_count == 0) {
+            EXPECT_EQ(event.event_path, "/tmp/testfile");
+            EXPECT_EQ(event.type, Core::FileWatcherEvent::Type::ChildCreated);
+        } else if (event_count == 1) {
+            EXPECT_EQ(event.event_path, "/tmp/testfile");
+            EXPECT_EQ(event.type, Core::FileWatcherEvent::Type::ChildDeleted);
+
+            event_loop.quit(0);
+        }
+
+        event_count++;
+    };
+
+    auto timer1 = Core::Timer::create_single_shot(500, [&] {
+        int rc = creat("/tmp/testfile", 0777);
+        EXPECT_NE(rc, -1);
+    });
+    timer1->start();
+
+    auto timer2 = Core::Timer::create_single_shot(1000, [&] {
+        int rc = unlink("/tmp/testfile");
+        EXPECT_NE(rc, -1);
+    });
+    timer2->start();
+
+    auto catchall_timer = Core::Timer::create_single_shot(2000, [&] {
+        VERIFY_NOT_REACHED();
+    });
+    catchall_timer->start();
+
+    event_loop.exec();
+}