r.proj: Add unit tests

[ninsbl]

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

[sudhanshu112233shukla]

Hi sir I am interested in working on this issue. Could you please assign it to me.

[Aterg264]

Hi! I want to contribute, and it's my first issue. Can you help me?
I don't quite understand what type of unit test is needed to validate data conversion between different CRS. In the essential unit testing documentation (https://grass.osgeo.org/grass-stable/manuals/libpython/gunittest_testing.html), there is information on how to create tests for GRASS modules, tests for C and C++ code, tests for Python code, and testing Python with doctest. The second option would be the correct one since it's a GRASS library written in C++. If that's the case, the test could be done in Python. Should I use gunittest as the first example of the documentation?