Add two regression tests for stdio over pipes.

test-stdio-pipe-access tests if accessing stdio pipe that is being read
by another process does not deadlocks Node.js. This was reported in
nodejs#10836 and was fixed in v8.3.0.
The deadlock would happen intermittently, so we run the test 5 times.

test-stdio-pipe-redirect tests if redirecting one child process stdin to
another process stdout does not crash Node as reported in
nodejs#17493. It was fixed in
nodejs#18019.