Add an optional prefix to tests

[greole]

For context see:

One quick note, we should add an optional prefix to test since we might have several files with the sameName.cpp which should not produce the same test. So if we just could add the possibility to add neofoam_unit_test(Name Domain finiteVolume) to produce a test Name_finiteVolume.

Originally posted by @greole in #112 (comment)