tests: (lsfd) avoid race conditions (part 1)

tests: (lsfd) avoid race conditions (part 1)

For terminating test_mkfds process, the test cases
used SIGCONT. test_mkfds used pselect for waiting for the signal.
After receiving the signal test_mkfds exited.

There is a race condition in this logic. If a test case sends SIGCONT
to test_mkfds process before the process calls pselect, the signal
may be lost. As the result, test_mkfds keeps waiting forever in pselect.

This change replaces the code sending SIGCONT with writing to the pipe
connecting to the stdin of test_mkfds. test_mkfds also monitors
its stdin in the pselect call. Unlike the signal, the written bytes to
the pipe are queued.

This change doesn't touch tests/ts/lsfd/option-inet and option-summary
test cases. They don't use coproc. So fixing the race condition in the test
cases can be much complicated. I will make a dedicated pull request (part 2)
for them.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>