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.

CMakeLists.txt

@@ -252,6 +252,7 @@ if(BUSTED_PRG)
   add_custom_target(functionaltest
     COMMAND ${CMAKE_COMMAND}
       -DBUSTED_PRG=${BUSTED_PRG}
+      -DNVIM_PRG=$<TARGET_FILE:nvim>
       -DWORKING_DIR=${CMAKE_CURRENT_SOURCE_DIR}
       -DBUSTED_OUTPUT_TYPE=${BUSTED_OUTPUT_TYPE}
       -DTEST_DIR=${CMAKE_CURRENT_SOURCE_DIR}/test

cmake/RunTests.cmake

@@ -1,5 +1,9 @@
 get_filename_component(BUSTED_DIR ${BUSTED_PRG} PATH)
 set(ENV{PATH} "${BUSTED_DIR}:$ENV{PATH}")
+if(NVIM_PRG)
+  set(ENV{NVIM_PROG} "${NVIM_PRG}")
+endif()
+
 if(DEFINED ENV{TEST_FILE})
   set(TEST_DIR $ENV{TEST_FILE})
 endif()