Tests: Fix compile errors on Clang

Since Clang enables a couple of warnings that we don't have in GCC,
these were not caught before. Included fixes:

- Use correct printf format string for `size_t`
- Don't compare Nonnull(Ref|Own)Ptr` to nullptr
- Fix unsigned int& => unsigned long& conversion

Tests/Kernel/fuzz-syscalls.cpp

@@ -134,7 +134,7 @@ static void do_random_tests()
         0xffffffff,
     };
     dbgln("Doing a few random syscalls with:");
-    for (unsigned long& interesting_value : interesting_values) {
+    for (const auto& interesting_value : interesting_values) {
         dbgln("  {0} ({0:p})", interesting_value);
     }
     for (size_t i = 0; i < fuzz_syscall_count; ++i) {