Recreate unit tests

[wleoncio]

The unit tests for the package have been simplified so the package could conform to CRAN policies. There is no need to completely remove them from the repository, as they only need to be excluded from the package itself. Having unit tests is very important in maintaining compatibility across versions.
Hence, I propose:

• Re-adding the Binder notebook content as unit tests
• Adding the unit tests to .Rbuildignore so they do not add overhead to the CRAN check