Commit 890c647e0f fixed an off-by-one bug, so the mapping of the page
at the very end of the user address space now works correctly.
This change adjusts the test so cover the corner cases the original
version was designed too.validate.
During a recent commit the 64-bit kernel was moved to a different
address, breaking this test (unnoticed). This fixes it, so we can
turn on breaking x86_64 tests on the CI again.
This test exposed a kernel panic in is_user_range calculations, so let's
convert it to be a LibTest test so we can prevent regressions in mmap,
the page allocator, and the memory manager.
