test_runner: use std::endl to ensure messages are flushed
test_runner: use std::endl to ensure messages are flushed

We've seen cases like
https://github.com/scylladb/scylla/issues/10830#issuecomment-1158899187
where the "random-seed=" message was missing from test logs
after an early-stage error.

I suspect ths happened since stdout wasn't flushed on
UndefinedBehaviorSanitizer termination.

This change converts '\n' printed to the logs
(std::cout or std::cerr) with std::endl, that
behaves as follows: (see https://en.cppreference.com/w/cpp/io/manip/endl)
> Inserts a newline character into the output sequence os and flushes it

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar