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.
Merge pull request #1395 from jszakmeister/fix-run-functional-tests

build: fix running of functional tests directly with CMake