pytest: Implement `fifo` based communication

[karthiknadig]

Why do we need to communicate like this?
Pytest reports tests as it discovers/runs it. we moved to this model so User has feedback on what is going on. For repos with > 10k tests some times the discovery process can take long and there is no indication on what is going on. We used socket communication to send back test status to update the UI.

Notes for anyone who wants to understand why this is complicated:

1. Stdio: pytest prints a lot of content to stdout, using stdout and stderr to parse useful information out is has error prone, and with plugins it can break easily.
2. file output: pytest has the option to write to files, but that also mean you don't get live feedback. Another issue is that plugins make sometimes do not follow convention, like the markdown test plugin. This again leads to error prone parsing, and no life test results.
3. TCP sockets: Plugins like the one mentioned here can block socket communication. Sockets can also be blocked due to filrewalls.
4. Named Pipes: On windows this creates actual named pipes, on linux/mac Node currently only has UDS sockets. FIFOs are not available out of the box. We are working on providing a FIFO based solution for unix, but there are implementation complications as FIFOs are handled differently be each flavor of linux. We also need to make sure that work when you do parallel runs of tests.

Things to consider:

• Should work with parallel runs
• Should work even is sockets are disabled
• Should report error when fifo is not available