test: relax TTY availability assumptions

test-tty.c currently assumes that a TTY is available to the test runner,
and fails hard if not. This may not be true on some autobuilding
environment, making the build fail as shown in [0].
Instead, let's properly skip the test in such cases.

[0] http://ur1.ca/fr5bd

Signed-off-by: Luca Bruno <lucab@debian.org>
test: relax timing assumption of loop_stop

The loop_stop test makes an implicit assumption about the triggering
timing of a repeating trigger, which may not hold true on slower or
virtualized machines, thus failing the test as shown at [0] and
discussed at [1].
This commit relaxes the assumption, without mandating the exact number
of runs.

[0] http://ur1.ca/fr5bw
[1] https://groups.google.com/d/msg/libuv/5-fNIC7hIAo/yqznDmwHDAIJ

Signed-off-by: Luca Bruno <lucab@debian.org>
test: relax timing assumption of timer_again

timer_again test makes an implicit assumption on the triggering
timing of a repeating timer. However, this assumption may be not
true on slower or virtualized architecture due to delay accumulation,
which may fail the test as show in [0].
This commit makes explicit checks conforming to the asserted behavior.

[0] http://ur1.ca/fr5c4

Signed-off-by: Luca Bruno <lucab@debian.org>