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
    )

src/nvim/testdir/test49.vim

@@ -456,7 +456,7 @@ function! ExtraVim(...)
     " messing up the user's viminfo file.
     let redirect = a:0 ?
 	\ " -c 'au VimLeave * redir END' -c 'redir\\! >" . a:1 . "'" : ""
-    exec "!echo '" . debug_quits . "q' | ../../../build/bin/nvim -u NONE -N -es" . redirect .
+    exec "!echo '" . debug_quits . "q' | $NVIM_PRG -u NONE -N -es" . redirect .
 	\ " -c 'debuggreedy|set viminfo+=nviminfo'" .
 	\ " -c 'let ExtraVimBegin = " . extra_begin . "'" .
 	\ " -c 'let ExtraVimResult = \"" . resultfile . "\"'" . breakpoints .