r.proj: Add unit tests

[ninsbl]

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Is your feature request related to a problem? Please describe. The external PROJ library has undergone many changes requireing plenty of follow up from GRASS devs. Since the role of PROJ is quite central the component should be covered with at least basic unit tests which are lacking as of now.

Describe the solution you'd like A simple, basic unittest with artificial data, converting a limited set of different CRS. The number of different tests and options to test can be increased also later (e.g. OpenMP, ...) ... Ideally, examples from the manual are used for testing...

Additional context See e.g.: #2502

[mukundsrinivasb]

I would like to contribute to this issue, since it seems relatively easy to wrap my head around. Please direct me to resource on which files I should commit to