Test: relax some implicit assumptions

The test-suite makes several assumptions on the build environment (mostly about tty availability and timers deadline) which may not be true under heavy-loaded autobuilders or slower/virtualized architectures.
This PR is target to the v0.10 stable branch and aims at fixing several occasional false-positive test-suite failures as experienced on Debian autobuilders, whose logs are available at https://buildd.debian.org/status/logs.php?pkg=libuv