Tests: Add a unit test to ensure the /dev/mem device works correctly

To ensure everything works as expected, a unit test was added with
multiple scenarios.
This binary has to have the SetUID flag, and we also bind-mount the
/usr/Tests directory to allow running of SetUID binaries.

Tests/Kernel/TestMemoryDeviceMmap.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2021, Liav A. <liavalb@hotmail.co.il>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Types.h>
+#include <LibTest/TestCase.h>
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+static ALWAYS_INLINE bool mem_chunk(int fd, u64 base, u64 length)
+{
+    u64 mmoffset = base % sysconf(_SC_PAGESIZE);
+    void* mmp = mmap(NULL, mmoffset + length, PROT_READ, MAP_SHARED, fd, base - mmoffset);
+    return mmp != MAP_FAILED;
+}
+
+TEST_CASE(test_memory_access_device_mmap)
+{
+    int rc = geteuid();
+    EXPECT_EQ(rc, 0);
+
+    int fd = open("/dev/mem", O_RDONLY);
+    EXPECT_EQ(fd < 0, false);
+
+    // FIXME: This is expected to work on QEMU machines (both 440FX and Q35),
+    // however, it will be much nicer to have some sort of a node in the ProcFS
+    // to expose physical memory ranges (e820 memory map).
+
+    auto result = mem_chunk(fd, 0xe0000, 0x100000 - 0xe0000);
+    EXPECT_EQ(result, true);
+
+    result = mem_chunk(fd, 0x100000, 0x200000 - 0x100000);
+    EXPECT_EQ(result, false);
+
+    result = mem_chunk(fd, 0xf0000, 70000);
+    EXPECT_EQ(result, false);
+
+    result = mem_chunk(fd, 0xfffc0000, 16384);
+    EXPECT_EQ(result, true);
+
+    result = mem_chunk(fd, 0xfffc0000, 0x100000);
+    EXPECT_EQ(result, false);
+}