test: delay child exit in AIX for pseudo-tty tests

The tests in pseudo-tty takes the form of child node writing some data and exiting, while parent python consume them through pseudo tty implementations, and validate the result.

While there is no synchronization between child and parent, this works for most platforms, except AIX, where the child exits even before the parent could setup the read loop, under race conditions

Fixing the race condition is ideally done through sending ACK messages to and forth, but involves massive changes and have side effect. The workaround is to address them in AIX alone, by adding a reasonable delay.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)