Make page diff in test_vm_bits more robust and nicer to read

[hlinnaka]

Suggestion from @koivunej:

I am increasingly confused about the 8:100 ... If that is not 8: because of assertion messages, shouldn't we just chunk the 8: to be 16 or 32 bytes and check those individually? Chunking & asserting would cause us to lose if the pages differ in two chunks.. Maybe doing a diff -u <(hexdump -Cv --skip 8 page_from_cache) <(hexdump -Cv --skip 8 page_from_ps) would be easiest and cleanest for the assertion message, and assert on the diff exit code?

Unsure if mac has compatible hexdump.

Originally posted by @koivunej in #7989 (comment)