Implement proper tests

[susilehtola]

There are no tests for now, but they should be implemented.

Basic tests:

• GTO and STO integrals / matrix elements
• Core guess energies
• Restricted and unrestricted calculations (code is the same, though!)

[loriab]

6 adds testing setup in the EFV model where one test builds atomtest exe and further tests execute it w/different args. you could expand that and add catch2 so it's actually checking numbers. Or you might be able to check for Converged to energy with something like the below.

set_tests_properties(smoke_test PROPERTIES
        PASS_REGULAR_EXPRESSION "^Hello, World!"
)