[RFC] build: fix running of functional tests directly with CMake

If you aren't just building everything into build/, then the functional
tests fail because they can't find the nvim executable.  Let's pass in
the location of the nvim executable, and set NVIM_PRG environment
variable accordingly.
Just out of curiosity, what does `$<TARGET_FILE:nvim>` do? Never seen this syntax before.

Otherwise, LGTM.