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/AK/TestNonnullRefPtr.cpp

@@ -16,7 +16,6 @@ struct Object : public RefCounted<Object> {
 TEST_CASE(basics)
 {
     auto object = adopt_ref(*new Object);
-    EXPECT(object.ptr() != nullptr);
     EXPECT_EQ(object->ref_count(), 1u);
     object->ref();
     EXPECT_EQ(object->ref_count(), 2u);