refs #18011 disable some newly failing tests on cpp windows; refs #17946 increase timeout for tchannels

[timotheecour]

refs #18011

(innocuous change to see if i can reproduce #18011 before disabling failing tests)
EDIT: indeed, I can

CI failures:

• osx tasynchttpserver_transferencoding fails because of tasynchttpserver_transferencoding.nim is flaky #17456
• windows tests/stdlib/tchannels.nim fails because of tests/stdlib/tchannels.nim flaky, gives reTimeout #17946
  thanks to testament :show duration also for failed tests; improve tshould_not_work; mitigate #17946 tchannels timeouts #17947 we now know why:

2021-05-14T20:20:29.9353041Z FAIL: tests/stdlib/tchannels.nim c (21.81 sec)

• windows tcpp_imported_exc.nim, tests/cpp/temitlist.nim, tests/cpp/tempty_generic_obj.nim fails because of nim cpp: tcpp_imported_exc, temitlist.nim, tempty_generic_obj.nim, recently fail on windows #18011

[timotheecour]

@Araq I'm merging this because #18011 affects all recently rebased PR's, the bug is kept open as the tests were disabled, not fixed (can't really debug the problem as I'm not on windows)

as you can see with 1st commit, the bug triggers for un-related reason; the next commits then disable the newly failing tests