misc: tst-poll: Proper synchronize with child before sending the signal
When running the testsuite in parallel, for instance running make -j
$(nproc) check, occasionally tst-epoll fails with a timeout. It happens
because it sometimes takes a bit more than 10ms for the process to get
cloned and blocked by the syscall. In that case the signal is
sent to early, and the test fails with a timeout.