Run tests in parallel inside GitHub actions

[rodsan0]

Makefile (more specifically make) has an argument that can be passed to run targets in parallel instead of sequentially. It achieves this by running the targets in batches. This means the behavior is similar to running them normally--if any of the tests in a batch fails, the batch stops gracefully and tests do not continue to be executed.

Ideally, the number of targets run in each batch is equal to the number of processes available in the CPU (can be obtained with nproc). As such, I think it would be a good idea that the multiple cd tests && make in .github/CI.yml be changed to cd tests && make -j$(nproc).

This is a good stop-gap change until CMake/CTest is used for testing in the future, as it would theoretically speed up GitHub actions by 100% (GH Actions has two processes available).