Mark a lot of flacky tests with TT_RETRIABLE (for linux/win32 only)

Mark a lot of flacky tests with TT_RETRIABLE (for linux/win32 only)

This patch mark testcases that only fail under travis-ci/appveyor with
TT_RETRIABLE, since otherwise there is too much noise, other issues
(like failures under vagrant boxes) would be investigated separatelly.

linux (from travis-ci only):
- http/cancel_by_host_no_ns
- http/cancel_by_host_inactive_server
- http/cancel_by_host_ns_timeout
- http/cancel_by_host_ns_timeout_inactive_server
- thread/conditions_simple
- util/monotonic_prc_precise
- util/usleep
- main/del_wait

vagrant/ubuntu box (this is the only exception):
- thread/no_events

win32 (from appveyor only):
- main/active_later
- main/persistent_active_timeout

And we should use TT_RETRIABLE over TT_OFF_BY_DEFAULT/TT_SKIP when it
make sense.

But there is still "test-ratelim__group_lim" left.