Add (failing) {In,Out}_channel linearization tests

[OlivierNicole]

Note: this depends on #5, which is why I open it as a draft PR.

This adds linearizability testing for a few functions of the new Stdlib modules In_channel and Out_channel. As of now these functions are not thread-safe, so the tests fails, sometimes with segfaults (due to concurrent close and output functions in Out_channel).

Left to do:

• Update dune file appropriately once the fate of Put Lin and STM libraries in a standalone package #5 is settled.
• Determine how this (currently failing) can be integrated properly into the test suite.
• Maybe enrich the test to cover more functions once their precise contract is decided by the compiler team (fix, or not fix and document, etc.)

[OlivierNicole]

Closing as this is now superseeded by #270.