Fortran Tests

[benegee]

Unfortunately Fortran is not natively supported by google test. A way around would be to write a Fortran program testing everything that is currently covered by the C-tests, write an interface to this function, and call it from a C++ google test.
It feels a little strange though, because we would call an interface function to Fortran, which would then call interface functions to C.

Alternatively, provided we keep running our examples, we could make sure that the examples use all of the provided Fortran functions.

The latter however seems not straightforward as apparently the Fortran interface calls are not considered in the coverage reports.

Originally posted by @bgeihe in #73 (comment)

[benegee]

There are some Fortran unit testing frameworks, as listed, e.g., here or here. However, I don't know any of them, so I can't give good recommendations.

We could also try to roll our own, e.g., to essentially code up 2-3 examples as tests, where we use the interface functions and verify the return values/functionality. Since we test the "external language <-> Julia" interaction already with the C code, we really only have to verify that the Fortran layer does not change anything while interacting with C, so I think it would be feasible to do it using manually coded tests.

Originally posted by @sloede in #73 (comment)