build: oldtest target: TEST_FILE, NVIM_PRG (#6098)

- Add support for TEST_FILE to the `oldtest` target, for consistency
  with the busted/lua tests.
  Caveat: with the busted/lua tests TEST_FILE takes a full path, whereas
  for `oldtest` it must be "test_foo.res".
- Add support for NVIM_PRG, again so that all test-related targets are
  consistent.
- Use consistent name for NVIM_PRG. But still need to support NVIM_PROG
  for QuickBuild CI.

Note: The `oldtest` target is driven by the top-level Makefile, because
it requires a TTY. CMake 3.2 added a USES_TERMINAL flag to
add_custom_target(). But we support CMake 2.8...

    add_custom_target(oldtest
      COMMAND make clean
      COMMAND make NVIM_PRG=$<TARGET_FILE:nvim> $ENV{MAKEOVERRIDES}
      DEPENDS nvim
      WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/src/nvim/testdir"
      USES_TERMINAL true
    )

test/functional/helpers.lua

@@ -17,7 +17,8 @@ local map = global_helpers.map
 local filter = global_helpers.filter
 
 local start_dir = lfs.currentdir()
-local nvim_prog = os.getenv('NVIM_PROG') or 'build/bin/nvim'
+-- XXX: NVIM_PROG takes precedence, QuickBuild sets it.
+local nvim_prog = os.getenv('NVIM_PROG') or os.getenv('NVIM_PRG') or 'build/bin/nvim'
 local nvim_argv = {nvim_prog, '-u', 'NONE', '-i', 'NONE', '-N',
                    '--cmd', 'set shortmess+=I background=light noswapfile noautoindent laststatus=1 undodir=. directory=. viewdir=. backupdir=.',
                    '--embed'}