ip6, escaped_string tests

[Kent Overstreet]

Hey Petr, here's the tests for ipv6 address and string_escape_mem(). I looked at
the clock code and it's just printing the name from a struct lookup - didn't
exactly seem worth a test.

There weren't any regressions from the printbuf patches found by these tests -
however, a recent fixup patch for prt_u64_minwidth() was wrong, I'll send you a
new version of that patch too.