[CMake] Never add current binary dir to command in ROOT_ADD_TEST

If a test added with `ROOT_ADD_TEST` uses a command that can't be found
on the system and that is not an absolute path, the `ROOT_ADD_TEST`
macro will implicitly prefix `$CMAKE_CURRENT_BINARY_DIR/`, assimum that
the executable is supposed to be found in the current binary dir.

However, that is not always a reasonable fallback, and can be even
wrong if the executable was supposed to be found in the `PATH` that is
set in the `ENVIRONMENT` for that test.

So I think it's better to remove that fallback, requiring the tests to
be explicit about which executable should be called.

Author: guitargeek

cmake/modules/RootMacros.cmake

@@ -1542,9 +1542,6 @@ function(ROOT_ADD_TEST test)
       find_program(_exe ${_prg})
       if(_exe)                                         # if the command is found in the system, use it
         set(_cmd ${_exe} ${ARG_COMMAND})
-      elseif(NOT IS_ABSOLUTE ${_prg})                  # if not absolute, assume is found in current binary dir
-        set(_prg ${CMAKE_CURRENT_BINARY_DIR}/${_prg})
-        set(_cmd ${_prg} ${ARG_COMMAND})
       else()                                           # take as it is
         set(_cmd ${_prg} ${ARG_COMMAND})
       endif()