MACAddress: Unit testing for basic functionality

Problem:
- There are no unit tests for `MACAddress` class. This makes it
  difficult to refactor and ensure the same behavior.
- `m_data` private member variable is uninitialized leading to undefined
  behavior of `is_zero()`.

Solution:
- Add unit tests to cover basic functionality.
- Initialize `m_data`.

AK/MACAddress.h

@@ -71,7 +71,7 @@ public:
     }
 
 private:
-    u8 m_data[6];
+    u8 m_data[6] {};
 };
 
 static_assert(sizeof(MACAddress) == 6);

AK/Tests/TestMACAddress.cpp

@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2020, 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/MACAddress.h>
+#include <AK/Types.h>
+
+TEST_CASE(should_default_construct)
+{
+    MACAddress sut {};
+    EXPECT(sut.is_zero());
+}
+
+TEST_CASE(should_braces_construct)
+{
+    MACAddress sut { 1, 2, 3, 4, 5, 6 };
+    EXPECT(!sut.is_zero());
+}
+
+TEST_CASE(should_construct_from_c_array)
+{
+    u8 addr[6] = { 1, 2, 3, 4, 5, 6 };
+    MACAddress sut(addr);
+    EXPECT(!sut.is_zero());
+}
+
+TEST_CASE(should_construct_from_6_octets)
+{
+    MACAddress sut(1, 2, 3, 4, 5, 6);
+    EXPECT(!sut.is_zero());
+}
+
+TEST_CASE(should_provide_access_to_octet_by_index)
+{
+    MACAddress sut(1, 2, 3, 4, 5, 6);
+    for (auto i = 0u; i < sizeof(MACAddress); ++i) {
+        EXPECT_EQ(i + 1, sut[i]);
+    }
+}
+
+TEST_CASE(should_equality_compare)
+{
+    MACAddress a(1, 2, 3, 4, 5, 6);
+    MACAddress b(1, 2, 3, 42, 5, 6);
+    EXPECT(a == a);
+    EXPECT(a != b);
+}
+
+TEST_CASE(should_string_format)
+{
+    MACAddress sut(1, 2, 3, 4, 5, 6);
+    EXPECT_EQ("01:02:03:04:05:06", sut.to_string());
+}
+
+TEST_MAIN(MACAddress)