🧪 Add simple test framework

Now that the project is finding a little adoption it is really high time
to bring some tests into the code to ensure everything works as it
should.
This first version will only provide a simple test harness from which
tests can slowly be written for other parts of the program.

test/test_helper/bats-assert

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+Subproject commit ffe84ea5dd43b568851549b3e241db150c12929c

test/test_helper/bats-support

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+Subproject commit 3c8fadc5097c9acfc96d836dced2bb598e48b009