SinglyLinkedList: Implement find in terms of AK::find

Problem:
- The implementation of `find` is coupled to the implementation of
  `SinglyLinkedList`.

Solution:
- Decouple the implementation of `find` from the class by using a
  generic `find` algorithm.

AK/Tests/TestSinglyLinkedList.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/TestSuite.h>
+
+#include <AK/SinglyLinkedList.h>
+
+static SinglyLinkedList<int> make_list()
+{
+    SinglyLinkedList<int> list {};
+    list.append(0);
+    list.append(1);
+    list.append(2);
+    list.append(3);
+    list.append(4);
+    list.append(5);
+    list.append(6);
+    list.append(7);
+    list.append(8);
+    list.append(9);
+    return list;
+}
+
+TEST_CASE(should_find_mutable)
+{
+    auto sut = make_list();
+
+    EXPECT_EQ(4, *sut.find(4));
+
+    EXPECT_EQ(sut.end(), sut.find(42));
+}
+
+TEST_CASE(should_find_mutable_with_predicate)
+{
+    auto sut = make_list();
+
+    EXPECT_EQ(4, *sut.find_if([](const auto v) { return v == 4; }));
+
+    EXPECT_EQ(sut.end(), sut.find_if([](const auto v) { return v == 42; }));
+}
+
+TEST_CASE(should_find_const)
+{
+    const auto sut = make_list();
+
+    EXPECT_EQ(4, *sut.find(4));
+
+    EXPECT_EQ(sut.end(), sut.find(42));
+}
+
+TEST_CASE(should_find_const_with_predicate)
+{
+    const auto sut = make_list();
+
+    EXPECT_EQ(4, *sut.find_if([](const auto v) { return v == 4; }));
+
+    EXPECT_EQ(sut.end(), sut.find_if([](const auto v) { return v == 42; }));
+}
+
+TEST_MAIN(SinglyLinkedList)