Add property based tests from multicoretests

[jmid]

This PR adds property-based tests from the multicoretests repository, now that qcheck-stm.0.1.0 has been released.

There are four files of property-based tests

• test/chan_stm_tests.ml contains a model-based STM test of Chan running both sequentially and in parallel
• test/task_parallel.ml contains tests for the different parallel for-loops
• test/task_one_dep.ml contains randomized task tests, where each task has only one dependency (in different pool variants)
• test/task_more_deps.ml contains randomized task tests, where each task can have several dependencies

The latter two generates a suitable random dependency graph represented as an array.

For context:

• 47 motivated developing these initial test/task_one_dep.ml tests

• the 2-workpool variant of test/task_one_dep.ml rediscovered #58
• test/task_parallel.ml found #67

[Sudha247]

Thanks @jmid!