Unit tests dont build

[Bharat Tak]

ubuntu 20.04
gcc 8.5

I updated catch.hpp like CelestiaProject/Celestia#1118 and then it worked.

[Wang Qi]

Thanks for the problem report.
I've update CATCH2 to v2.13.9.
Let me know if it works.

[Bharat Tak]

Thanks for the quick action. Ill test it and reply.

[Bharat Tak]

Works