Fix test_timers and test_channel flakiness

[ychin]

test_timers and test_channel tests occasionally fail in Travis. #782 turned them off in MacVim to avoid them breaking CI, but that's a temporary solution. Investigate why and fix it.

Edit: There are more tests that are broken and manually disabled in MacVim now. Need to go through them too.

[ychin]

#838 made the flaky tests not fail the whole test, but the issue is still happening.