Alcotest Test Suite

How to add new tests:

Create a new .ml file containing the tests (or add unit tests to one of the existing test .ml files)
include the name of the test module in all_tests.ml
- e.g. if the name of the new test module is test_deriver, put the following in all_tests.ml:
```
include Test_deriver
...
```
Then, invoke the newly-added test functions in test_runner.ml, which executes all the Alcotest test cases defined in this directory
Note: boilerplate.ml contains Alcotest utilities (e.g. testable instances) to facilitate writing unit tests
- The mk_test function in particular simplifies the process of writing Alcotest test cases

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